Documentation: Add binding document for Lattice iCE40 FPGA manager

This adds documentation of the device tree bindings of the Lattice iCE40
FPGA driver for the FPGA manager framework.
Signed-off-by: Joel Holdsworth <joel@airwebreathe.org.uk>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Alan Tull <atull@opensource.altera.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/fpga/lattice-ice40-fpga-mgr.txt

+Lattice iCE40 FPGA Manager
+
+Required properties:
+- compatible:		Should contain "lattice,ice40-fpga-mgr"
+- reg:			SPI chip select
+- spi-max-frequency:	Maximum SPI frequency (>=1000000, <=25000000)
+- cdone-gpios:		GPIO input connected to CDONE pin
+- reset-gpios:		Active-low GPIO output connected to CRESET_B pin. Note
+			that unless the GPIO is held low during startup, the
+			FPGA will enter Master SPI mode and drive SCK with a
+			clock signal potentially jamming other devices on the
+			bus until the firmware is loaded.
+
+Example:
+	fpga: fpga@0 {
+		compatible = "lattice,ice40-fpga-mgr";
+		reg = <0>;
+		spi-max-frequency = <1000000>;
+		cdone-gpios = <&gpio 24 GPIO_ACTIVE_HIGH>;
+		reset-gpios = <&gpio 22 GPIO_ACTIVE_LOW>;
+	};