fpga: add new symbol for fpga_loadb

fpga: add new symbol for fpga_loadb

Adding new symbol for the fpga loadb command which is exclusive to
Xilinx. Default value is y for backward compatibility.

Clarify the type of file used for fpga programming commands

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@amd.com>
Link: https://lore.kernel.org/r/20250121130138.1999916-6-ibai.erkiaga-elorza@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>