spi: Add Spreadtrum SPI controller documentation

Spreadtrum SPI Controller

Required properties:

- compatible: Should be "sprd,sc9860-spi".

- reg: Offset and length of SPI controller register space.

- interrupts: Should contain SPI interrupt.

- clock-names: Should contain following entries:

	"spi" for SPI clock,

	"source" for SPI source (parent) clock,

	"enable" for SPI module enable clock.

- clocks: List of clock input name strings sorted in the same order

	as the clock-names property.

- #address-cells: The number of cells required to define a chip select

	address on the SPI bus. Should be set to 1.

- #size-cells: Should be set to 0.

Example:

spi0: spi@70a00000{

	compatible = "sprd,sc9860-spi";

	reg = <0 0x70a00000 0 0x1000>;

	interrupts = <GIC_SPI 7 IRQ_TYPE_LEVEL_HIGH>;

	clock-names = "spi", "source","enable";

	clocks = <&clk_spi0>, <&ext_26m>, <&clk_ap_apb_gates 5>;

	#address-cells = <1>;

	#size-cells = <0>;

};