fpga: zynqmp: Add secure bitstream loading for ZynqMP

This patch adds support for loading secure bitstreams on ZynqMP
platforms. The secure bitstream images has to be generated using
Xilinx bootgen tool.

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