arm64: zynqmp: Add support for verifying secure images

This patch adds new command "zynqmp" to handle zynqmp
specific commands like "zynqmp secure". This secure command is
used for verifying zynqmp specific secure images. The secure
image can either be authenticated or encrypted or both encrypted
and authenticated. The secure image is prepared using bootgen
and will be in xilinx specific BOOT.BIN format. The optional
key can be used for decryption of encrypted image if user
key was specified while creation BOOT.BIN.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>