spi: dw: document device tree binding

Synopsys DesignWare SPI master

Required properties:

- compatible: should be "snps,designware-spi"

- #address-cells: see spi-bus.txt

- #size-cells: see spi-bus.txt

- reg: address and length of the spi master registers

- interrupts: should contain one interrupt

- clocks: spi clock phandle

- num-cs: see spi-bus.txt

Optional properties:

- cs-gpios: see spi-bus.txt

Example:

spi: spi@4020a000 {

	compatible = "snps,designware-spi";

	interrupts = <11 1>;

	reg = <0x4020a000 0x1000>;

	clocks = <&pclk>;

	num-cs = <2>;

	cs-gpios = <&banka 0 0>;

};