pci: Add board_ prefix to should_load_oprom() and make it weak

For consistency with board_should_run_oprom(), do the same to
should_load_oprom(). Board support codes can provide this one
to override the default weak one.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>