spi: add DT bindings for UniPhier SPI controller

Add DT bindings for SPI controller implemented in UniPhier SoCs.
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Keiji Hayashibara <hayashibara.keiji@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-uniphier.txt

+Socionext UniPhier SPI controller driver
+
+UniPhier SoCs have SCSSI which supports SPI single channel.
+
+Required properties:
+ - compatible: should be "socionext,uniphier-scssi"
+ - reg: address and length of the spi master registers
+ - #address-cells: must be <1>, see spi-bus.txt
+ - #size-cells: must be <0>, see spi-bus.txt
+ - clocks: A phandle to the clock for the device.
+ - resets: A phandle to the reset control for the device.
+
+Example:
+
+spi0: spi@54006000 {
+	compatible = "socionext,uniphier-scssi";
+	reg = <0x54006000 0x100>;
+	#address-cells = <1>;
+	#size-cells = <0>;
+	clocks = <&peri_clk 11>;
+	resets = <&peri_rst 11>;
+};