QCOM Idle States for cpuidle driver

ARM provides idle-state node to define the cpuidle states, as defined in [1].
cpuidle-qcom is the cpuidle driver for Qualcomm SoCs and uses these idle
states. Idle states have different enter/exit latency and residency values.
The idle states supported by the QCOM SoC are defined as -

    * Standby
    * Retention
    * Standalone Power Collapse (Standalone PC or SPC)
    * Power Collapse (PC)

Standby: Standby does a little more in addition to architectural clock gating.
When the WFI instruction is executed the ARM core would gate its internal
clocks. In addition to gating the clocks, QCOM cpus use this instruction as a
trigger to execute the SPM state machine. The SPM state machine waits for the
interrupt to trigger the core back in to active. This triggers the cache
hierarchy to enter standby states, when all cpus are idle. An interrupt brings
the SPM state machine out of its wait, the next step is to ensure that the
cache hierarchy is also out of standby, and then the cpu is allowed to resume
execution. This state is defined as a generic ARM WFI state by the ARM cpuidle
driver and is not defined in the DT. The SPM state machine should be
configured to execute this state by default and after executing every other
state below.

Retention: Retention is a low power state where the core is clock gated and
the memory and the registers associated with the core are retained. The
voltage may be reduced to the minimum value needed to keep the processor
registers active. The SPM should be configured to execute the retention
sequence and would wait for interrupt, before restoring the cpu to execution
state. Retention may have a slightly higher latency than Standby.

Standalone PC: A cpu can power down and warmboot if there is a sufficient time
between the time it enters idle and the next known wake up. SPC mode is used
to indicate a core entering a power down state without consulting any other
cpu or the system resources. This helps save power only on that core.  The SPM
sequence for this idle state is programmed to power down the supply to the
core, wait for the interrupt, restore power to the core, and ensure the
system state including cache hierarchy is ready before allowing core to
resume. Applying power and resetting the core causes the core to warmboot
back into Elevation Level (EL) which trampolines the control back to the
kernel. Entering a power down state for the cpu, needs to be done by trapping
into a EL. Failing to do so, would result in a crash enforced by the warm boot
code in the EL for the SoC. On SoCs with write-back L1 cache, the cache has to
be flushed in s/w, before powering down the core.

Power Collapse: This state is similar to the SPC mode, but distinguishes
itself in that the cpu acknowledges and permits the SoC to enter deeper sleep
modes. In a hierarchical power domain SoC, this means L2 and other caches can
be flushed, system bus, clocks - lowered, and SoC main XO clock gated and
voltages reduced, provided all cpus enter this state.  Since the span of low
power modes possible at this state is vast, the exit latency and the residency
of this low power mode would be considered high even though at a cpu level,
this essentially is cpu power down. The SPM in this state also may handshake
with the Resource power manager (RPM) processor in the SoC to indicate a
complete application processor subsystem shut down.

The idle-state for QCOM SoCs are distinguished by the compatible property of
the idle-states device node.

The devicetree representation of the idle state should be -

Required properties:

- compatible: Must be one of -
			"qcom,idle-state-ret",
			"qcom,idle-state-spc",
			"qcom,idle-state-pc",
		and "arm,idle-state".

Other required and optional properties are specified in [1].

Example:

	idle-states {
		CPU_SPC: spc {
			compatible = "qcom,idle-state-spc", "arm,idle-state";
			entry-latency-us = <150>;
			exit-latency-us = <200>;
			min-residency-us = <2000>;
		};
	};

[1]. Documentation/devicetree/bindings/arm/idle-states.yaml