SPI-GPIO devicetree bindings

Required properties:

 - compatible: should be set to "spi-gpio"
 - #address-cells: should be set to <0x1>
 - ranges
 - gpio-sck: GPIO spec for the SCK line to use
 - gpio-miso: GPIO spec for the MISO line to use
 - gpio-mosi: GPIO spec for the MOSI line to use
 - cs-gpios: GPIOs to use for chipselect lines
 - num-chipselects: number of chipselect lines

Example:

	spi {
		compatible = "spi-gpio";
		#address-cells = <0x1>;
		ranges;

		gpio-sck = <&gpio 95 0>;
		gpio-miso = <&gpio 98 0>;
		gpio-mosi = <&gpio 97 0>;
		cs-gpios = <&gpio 125 0>;
		num-chipselects = <1>;

		/* clients */
	};