dt-bindings: firmware: Add bindings for xilinx firmware

Add documentation to describe Xilinx firmware driver bindings.
Firmware driver provides an interface to firmware APIs.
Interface APIs can be used by any driver to communicate
to Platform Management Unit.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Link: https://lore.kernel.org/r/20210626155248.5004-4-nava.manne@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>