event: Add fpga load event

This enables implementing custom logic after a bitstream was loaded
into the fpga.

Signed-off-by: Christian Taedcke <christian.taedcke@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Link: https://lore.kernel.org/r/20230720072724.11516-1-christian.taedcke-oss@weidmueller.com


Signed-off-by: Michal Simek <michal.simek@amd.com>