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
 - interrupts: a single interrupt specifier
 - pinctrl-names: should be "default"
 - pinctrl-0: pin control state for the default mode
 - 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>;
	interrupts = <0 39 4>;
	pinctrl-names = "default";
	pinctrl-0 = <&pinctrl_spi0>;
	clocks = <&peri_clk 11>;
	resets = <&peri_rst 11>;
};