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

Xilinx Zynq QSPI controller Device Tree Bindings

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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>;

	};