spi: st: Provide Device Tree binding documentation

STMicroelectronics SSC (SPI) Controller

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Required properties:

- compatible	: "st,comms-ssc4-spi"

- reg		: Offset and length of the device's register set

- interrupts	: The interrupt specifier

- clock-names	: Must contain "ssc"

- clocks	: Must contain an entry for each name in clock-names

		    See ../clk/*

- pinctrl-names	: Uses "default", can use "sleep" if provided

		    See ../pinctrl/pinctrl-binding.txt

Optional properties:

- cs-gpios	: List of GPIO chip selects

		    See ../spi/spi-bus.txt

Child nodes represent devices on the SPI bus

  See ../spi/spi-bus.txt

Example:

	spi@9840000 {

		compatible	= "st,comms-ssc4-spi";

		reg		= <0x9840000 0x110>;

		interrupts	= <GIC_SPI 112 IRQ_TYPE_LEVEL_HIGH>;

		clocks		= <&clk_s_c0_flexgen CLK_EXT2F_A9>;

		clock-names	= "ssc";

		pinctrl-0	= <&pinctrl_spi0_default>;

		pinctrl-names	= "default";

		cs-gpios	= <&pio17 5 0>;

		#address-cells	= <1>;

		#size-cells	= <0>;

		st95hf@0{

			compatible		= "st,st95hf";

			reg			= <0>;

			spi-max-frequency	= <1000000>;

			interrupts		= <2 IRQ_TYPE_EDGE_FALLING>;

		};

	};