Broadcom BCM2835 auxiliar SPI1/2 controller

The BCM2835 contains two forms of SPI master controller, one known simply as
SPI0, and the other known as the "Universal SPI Master"; part of the
auxiliary block. This binding applies to the SPI1/2 controller.

Required properties:
- compatible: Should be "brcm,bcm2835-aux-spi".
- reg: Should contain register location and length for the spi block
- interrupts: Should contain shared interrupt of the aux block
- clocks: The clock feeding the SPI controller - needs to
	  point to the auxiliar clock driver of the bcm2835,
	  as this clock will enable the output gate for the specific
	  clock.
- cs-gpios: the cs-gpios (native cs is NOT supported)
	    see also spi-bus.txt

Example:

spi1@7e215080 {
	compatible = "brcm,bcm2835-aux-spi";
	reg = <0x7e215080 0x40>;
	interrupts = <1 29>;
	clocks = <&aux_clocks BCM2835_AUX_CLOCK_SPI1>;
	#address-cells = <1>;
	#size-cells = <0>;
	cs-gpios = <&gpio 18>, <&gpio 17>, <&gpio 16>;
};

spi2@7e2150c0 {
	compatible = "brcm,bcm2835-aux-spi";
	reg = <0x7e2150c0 0x40>;
	interrupts = <1 29>;
	clocks = <&aux_clocks BCM2835_AUX_CLOCK_SPI2>;
	#address-cells = <1>;
	#size-cells = <0>;
	cs-gpios = <&gpio 43>, <&gpio 44>, <&gpio 45>;
};