dt-bindings: firmware: Add bindings for ZynqMP firmware

Add documentation to describe Xilinx ZynqMP firmware driver
bindings. Firmware driver provides an interface to firmware
APIs. Interface APIs can be used by any driver to communicate
to PMUFW (Platform Management Unit).
Signed-off-by: Rajan Vaja <rajanv@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/firmware/xilinx/xlnx,zynqmp-firmware.txt

+-----------------------------------------------------------------
+Device Tree Bindings for the Xilinx Zynq MPSoC Firmware Interface
+-----------------------------------------------------------------
+
+The zynqmp-firmware node describes the interface to platform firmware.
+ZynqMP has an interface to communicate with secure firmware. Firmware
+driver provides an interface to firmware APIs. Interface APIs can be
+used by any driver to communicate to PMUFW(Platform Management Unit).
+These requests include clock management, pin control, device control,
+power management service, FPGA service and other platform management
+services.
+
+Required properties:
+ - compatible:	Must contain:	"xlnx,zynqmp-firmware"
+ - method:	The method of calling the PM-API firmware layer.
+		Permitted values are:
+		  - "smc" : SMC #0, following the SMCCC
+		  - "hvc" : HVC #0, following the SMCCC
+
+-------
+Example
+-------
+
+firmware {
+	zynqmp_firmware: zynqmp-firmware {
+		compatible = "xlnx,zynqmp-firmware";
+		method = "smc";
+	};
+};