spi: add DT bindings for UniPhier SPI controller

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>;

};