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Keystone 2 DSP GPIO controller bindings

HOST OS userland running on ARM can send interrupts to DSP cores using
the DSP GPIO controller IP. It provides 28 IRQ signals per each DSP core.
This is one of the component used by the IPC mechanism used on Keystone SOCs.

For example TCI6638K2K SoC has 8 DSP GPIO controllers:
 - 8 for C66x CorePacx CPUs 0-7

Keystone 2 DSP GPIO controller has specific features:
- each GPIO can be configured only as output pin;
- setting GPIO value to 1 causes IRQ generation on target DSP core;
- reading pin value returns 0 - if IRQ was handled or 1 - IRQ is still
  pending.

Required Properties:
- compatible: should be "ti,keystone-dsp-gpio"
- ti,syscon-dev: phandle/offset pair. The phandle to syscon used to
  access device state control registers and the offset of device's specific
  registers within device state control registers range.
- gpio-controller: Marks the device node as a gpio controller.
- #gpio-cells: Should be 2.

Please refer to gpio.txt in this directory for details of the common GPIO
bindings used by client devices.

Example:
	dspgpio0: keystone_dsp_gpio@2620240 {
		compatible = "ti,keystone-dsp-gpio";
		ti,syscon-dev = <&devctrl 0x240>;
		gpio-controller;
		#gpio-cells = <2>;
	};

	dsp0: dsp0 {
		compatible = "linux,rproc-user";
		...
		kick-gpio = <&dspgpio0 27>;
	};