dt-bindings: firmware: Add bindings for Versal firmware

ZynqMP firmware driver can be used for versal also.
Add versal compatible string to zynqmp firmware driver
doc.

Signed-off-by: Jolly Shah <jolly.shah@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>