doc: device-tree-bindings: spi: Add zynq qspi info

Added device-tree-binding information for zynq qspi controller
driver.

Signed-off-by: Jagan Teki <jteki@openedev.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Jagan Teki <jteki@openedev.com>

doc/device-tree-bindings/spi/spi-zynq-qspi.txt

@@ -0,0 +1,26 @@
+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.
+- 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@e000d000 {
+		compatible = "xlnx,zynq-qspi-1.0";
+		clock-names = "ref_clk", "pclk";
+		clocks = <&clkc 10>, <&clkc 43>;
+		interrupt-parent = <&intc>;
+		interrupts = <0 19 4>;
+		num-cs = <1>;
+		reg = <0xe000d000 0x1000>;
+	} ;