pci: Make load_oprom and run_oprom independent

At present should_load_oprom() calls board_should_run_oprom() to
determine whether oprom should be loaded. But sometimes we just
want to load oprom without running. Make them independent.

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