cmd: fpga: Add support to load secure bitstreams

This patch adds support to load secure bitstreams(authenticated or
encrypted or both). As of now, this feature is added and tested only
for xilinx bitstreams and the secure bitstream was generated using
xilinx bootgen tool, but the command is defined in more generic way.

Command example to load authenticated and device key
encrypted bitstream is as follows
"fpga loads 0 100000 2000000 0 1"

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