Zynq SPI controller Device Tree Bindings
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Required properties:
- compatible		: Should be "xlnx,spi-zynq".
- reg			: Physical base address and size of SPI registers map.
- status		: Status will be disabled in dtsi and enabled in required dts.
- interrupt-parent	: Must be core interrupt controller.
- interrupts		: Property with a value describing the interrupt
			  number.
- clocks		: Clock phandles (see clock bindings for details).
- clock-names		: List of input clock names - "ref_clk", "pclk"
			  (See clock bindings for details).
- spi-max-frequency	: Maximum SPI clocking speed of device in Hz

Example:

	spi@e0006000 {
		compatible = "xlnx,zynq-spi";
		reg = <0xe0006000 0x1000>;
		status = "disabled";
		interrupt-parent = <&intc>;
		interrupts = <0 26 4>;
		clocks = <&clkc 25>, <&clkc 34>;
		clock-names = "ref_clk", "pclk";
		spi-max-frequency = <166666700>;
		#address-cells = <1>;
		#size-cells = <0>;
	} ;