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

Add documentation to describe Xilinx ZynqMP nvmem driver
bindings.
Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/nvmem/xlnx,zynqmp-nvmem.txt

+--------------------------------------------------------------------------
+=  Zynq UltraScale+ MPSoC nvmem firmware driver binding =
+--------------------------------------------------------------------------
+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
+
+-------
+ Example
+-------
+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";
+
+		...
+	};