dt: bindings: fpga: add lattice machxo2 slave spi binding description

Add dt binding documentation details for Lattice MachXO2 FPGA configuration
over Slave SPI interface.
Signed-off-by: Paolo Pisati <p.pisati@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/fpga/lattice-machxo2-spi.txt

+Lattice MachXO2 Slave SPI FPGA Manager
+
+Lattice MachXO2 FPGAs support a method of loading the bitstream over
+'slave SPI' interface.
+
+See 'MachXO2ProgrammingandConfigurationUsageGuide.pdf' on www.latticesemi.com
+
+Required properties:
+- compatible: should contain "lattice,machxo2-slave-spi"
+- reg: spi chip select of the FPGA
+
+Example for full FPGA configuration:
+
+	fpga-region0 {
+		compatible = "fpga-region";
+		fpga-mgr = <&fpga_mgr_spi>;
+		#address-cells = <0x1>;
+		#size-cells = <0x1>;
+	};
+
+	spi1: spi@2000 {
+        ...
+
+		fpga_mgr_spi: fpga-mgr@0 {
+			compatible = "lattice,machxo2-slave-spi";
+			spi-max-frequency = <8000000>;
+			reg = <0>;
+		};
+	};