spi: st: Provide Device Tree binding documentation

This patch adds DT documentation for the SPI portion of ST's SSC device.
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-st-ssc.txt

+STMicroelectronics SSC (SPI) Controller
+---------------------------------------
+
+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>;
+		};
+	};