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>