dt-bindings: spi: Add device tree binding documentation for Zynq QSPI controller

This patch adds the dts binding document for Zynq SOC QSPI controller.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Naga Sureshkumar Relli <naga.sureshkumar.relli@xilinx.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

+Xilinx Zynq QSPI controller Device Tree Bindings
+-------------------------------------------------------------------
+
+Required properties:
+- compatible		: Should be "xlnx,zynq-qspi-1.0".
+- reg			: Physical base address and size of QSPI registers map.
+- interrupts		: Property with a value describing the interrupt
+			  number.
+- 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@e000d000 {
+		compatible = "xlnx,zynq-qspi-1.0";
+		reg = <0xe000d000 0x1000>;
+		interrupt-parent = <&intc>;
+		interrupts = <0 19 4>;
+		clock-names = "ref_clk", "pclk";
+		clocks = <&clkc 10>, <&clkc 43>;
+		num-cs = <1>;
+	};