fpga: Guard the LOADMK functionality with CMD_FPGA_LOADMK

Guard the LOADMK functionality with config to provide
an option to enable or disable it.
Enable it for all platforms in mainline which enable CONFIG_CMD_FPGA.

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