Add documentation to describe Xilinx ZynqMP AES-GCM driver bindings.

Signed-off-by: Kalyani Akula <kalyani.akula@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>