dt-bindings: nvmem: Add bindings for ZynqMP nvmem driver

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=  Zynq UltraScale+ MPSoC nvmem firmware driver binding =

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The nvmem_firmware node provides access to the hardware related data

like soc revision, IDCODE... etc, By using the firmware interface.

Required properties:

- compatible: should be "xlnx,zynqmp-nvmem-fw"

= Data cells =

Are child nodes of silicon id, bindings of which as described in

bindings/nvmem/nvmem.txt

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 Example

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firmware {

	zynqmp_firmware: zynqmp-firmware {

		compatible = "xlnx,zynqmp-firmware";

		method = "smc";

		nvmem_firmware {

			compatible = "xlnx,zynqmp-nvmem-fw";

			#address-cells = <1>;

			#size-cells = <1>;

			/* Data cells */

			soc_revision: soc_revision {

				reg = <0x0 0x4>;

			};

		};

	};

};

= Data consumers =

Are device nodes which consume nvmem data cells.

For example:

	pcap {

		...

		nvmem-cells = <&soc_revision>;

		nvmem-cell-names = "soc_revision";

		...

	};