x86: coreboot: make it possible to process unhandled tags

coreboot makes it possible to add own entries into coreboot's
table at a per mainboard basis. As there might be some custom
ones it makes sense to provide a way to process them.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>