x86: Support pci bus scan in the early phase

On x86, some peripherals on pci buses need to be accessed in the
early phase (eg: pci uart) with a valid pci memory/io address,
thus scan the pci bus and do the corresponding resource allocation.

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