fpga: zynqpl: Correct PL bitstream loading sequence for zynqaes

fpga: zynqpl: Correct PL bitstream loading sequence for zynqaes

Correct the PL bitstream loading sequence for zynqaes command by
clearing the loaded PL bitstream before loading the new encrypted
bitstream using the zynq aes command. This was done by setting
the PROG_B same as in case of fpgaload commands.
This patch fixes the issue of loading the encrypted PL bitstream
onto the PL in which a bitstream has already been loaded
successfully.

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