cmd: zynqrsa: Added support to load non-encrypted bitstream

cmd: zynqrsa: Added support to load non-encrypted bitstream

zynqrsa programs the PL when a partition includes an authenticated
and encrypted bitstream. However, if the partition is only
authenticated there is no PL programming operation because
there was no support to load only authenticated bitstream.

This patch added support to load only authenticated bitstream.

Signed-off-by: Vipul Kumar <vipul.kumar@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>