fpga: define dummy fpga_load function for debug build

fpga: define dummy fpga_load function for debug build

This fixes below build error when CC_OPTIMIZE_FOR_DEBUG is enabled and
CONFIG_FPGA or CONFIG_SPL_FPGA are not enabled.
When CC_OPTIMIZE_FOR_DEBUG is enabled, unused code will not be optimized
out. Hence, fpga_load function must have a dummy implementation to avoid
the build error.

../common/spl/spl_fit.c:591: undefined reference to `fpga_load'
collect2: error: ld returned 1 exit status

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Link: https://lore.kernel.org/r/20230831075247.137501-1-chanho61.park@samsung.com
Signed-off-by: Michal Simek <michal.simek@amd.com>