ICE40 FPGA based SPI-USB bridge

Documentation/devicetree/bindings/spi/spi-bus.txt provides the details
of the required and optional properties of a SPI slave device node.

The purpose of this document is to provide the additional properties
that are required to use the ICE40 FPGA based SPI slave device as a
USB host controller.

Required properties:
- compatible : should be "lattice,ice40-spi-usb"
- <supply-name>-supply: handle to the regulator device tree node
  Required "supply-name" is "core-vcc" and "spi-vcc"
- reset-gpio: gpio used to assert the bridge chip reset
- slave-select-gpio: gpio used to select the slave during configuration
  loading
- config-done-gpio: gpio used to indicate the configuration status
- vcc-en-gpio: gpio used to enable the chip power supply

Optional properties:
- interrupts: IRQ lines used by this controller
- clk-en-gpio: gpio used to enable the 19.2 MHZ clock to the bridge
  chip. If it is not present, assume that the clock is available on
  the bridge chip board.
- <supply-name>-supply: handle to the regulator device tree node
  Optional "supply-name" is "gpio" used to power up the gpio bank
  used by this device

	spi@f9923000 {
		lattice,spi-usb@3 {
			compatible = "lattice,ice40-spi-usb";
			reg = <3>;
			spi-max-frequency = <50000000>;
			spi-cpol = <1>;
			spi-cpha = <1>;
			interrupt-parent = <&msmgpio>;
			interrupts = <121 0x8>;
			core-vcc-supply = <&pm8226_l2>;
			spi-vcc-supply = <&pm8226_l5>;
			lattice,reset-gpio = <&msmgpio 114 0>;
			lattice,slave-select-gpio = <&msmgpio 118 0>;
			lattice,config-done-gpio = <&msmgpio 115 0>;
			lattice,vcc-en-gpio = <&msmgpio 117 0>;
		};
	};