dt: bindings: fpga: add xilinx slave-serial binding description

Add dt binding documentation details for Xilinx FPGA configuration
over slave serial interface.
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/fpga/xilinx-slave-serial.txt

+Xilinx Slave Serial SPI FPGA Manager
+
+Xilinx Spartan-6 FPGAs support a method of loading the bitstream over
+what is referred to as "slave serial" interface.
+The slave serial link is not technically SPI, and might require extra
+circuits in order to play nicely with other SPI slaves on the same bus.
+
+See https://www.xilinx.com/support/documentation/user_guides/ug380.pdf
+
+Required properties:
+- compatible: should contain "xlnx,fpga-slave-serial"
+- reg: spi chip select of the FPGA
+- prog_b-gpios: config pin (referred to as PROGRAM_B in the manual)
+- done-gpios: config status pin (referred to as DONE in the manual)
+
+Example for full FPGA configuration:
+
+	fpga-region0 {
+		compatible = "fpga-region";
+		fpga-mgr = <&fpga_mgr_spi>;
+		#address-cells = <0x1>;
+		#size-cells = <0x1>;
+	};
+
+	spi1: spi@10680 {
+		compatible = "marvell,armada-xp-spi", "marvell,orion-spi";
+		pinctrl-0 = <&spi0_pins>;
+		pinctrl-names = "default";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		cell-index = <1>;
+		interrupts = <92>;
+		clocks = <&coreclk 0>;
+		status = "okay";
+
+		fpga_mgr_spi: fpga-mgr@0 {
+			compatible = "xlnx,fpga-slave-serial";
+			spi-max-frequency = <60000000>;
+			spi-cpha;
+			reg = <0>;
+			done-gpios = <&gpio0 9 GPIO_ACTIVE_HIGH>;
+			prog_b-gpios = <&gpio0 29 GPIO_ACTIVE_LOW>;
+		};
+	};