spi: zynq: Add DT bindings documentation for Zynq Ultrascale+ MPSoC GQSPI controller

Add bindings documentation for GQSPI controller driver used by
Zynq Ultrascale+ MPSoC
Signed-off-by: NRanjit Waghmode <ranjit.waghmode@xilinx.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-zynqmp-qspi.txt

+Xilinx Zynq UltraScale+ MPSoC GQSPI controller Device Tree Bindings
+-------------------------------------------------------------------
+
+Required properties:
+- compatible		: Should be "xlnx,zynqmp-qspi-1.0".
+- reg			: Physical base address and size of GQSPI registers map.
+- interrupts		: Property with a value describing the interrupt
+			  number.
+- interrupt-parent	: Must be core interrupt controller.
+- clock-names		: List of input clock names - "ref_clk", "pclk"
+			  (See clock bindings for details).
+- clocks		: Clock phandles (see clock bindings for details).
+
+Optional properties:
+- num-cs		: Number of chip selects used.
+
+Example:
+	qspi: spi@ff0f0000 {
+		compatible = "xlnx,zynqmp-qspi-1.0";
+		clock-names = "ref_clk", "pclk";
+		clocks = <&misc_clk &misc_clk>;
+		interrupts = <0 15 4>;
+		interrupt-parent = <&gic>;
+		num-cs = <1>;
+		reg = <0x0 0xff0f0000 0x1000>,<0x0 0xc0000000 0x8000000>;
+	};