cmd: fpga: Update loads command help to handle devicekey support

cmd: fpga: Update loads command help to handle devicekey support

This patch updates the fpga loads command help to handle a case
where user wants to use devicekey for loading encrypted bitstreams
instead of userkey. The key address and size should be sent as
0xFFFFFFFF to let fpga driver to use devicekey.

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