SPI-GPIO devicetree bindings

This represents a group of 3-n GPIO lines used for bit-banged SPI on dedicated
GPIO lines.

Required properties:

 - compatible: should be set to "spi-gpio"
 - #address-cells: should be set to <0x1>
 - ranges
 - sck-gpios: GPIO spec for the SCK line to use
 - miso-gpios: GPIO spec for the MISO line to use
 - mosi-gpios: GPIO spec for the MOSI line to use
 - cs-gpios: GPIOs to use for chipselect lines.
             Not needed if num-chipselects = <0>.
 - num-chipselects: Number of chipselect lines. Should be <0> if a single device
                    with no chip select is connected.

Deprecated bindings:

These legacy GPIO line bindings can alternatively be used to define the
GPIO lines used, they should not be used in new device trees.

 - 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

Example:

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

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

		/* clients */
	};