dm: pci: Support selected device/driver binding before relocation

On some platforms pci devices behind bridge need to be probed (eg:
a pci uart on recent x86 chipset) before relocation. But we won't
bind all devices found during the enumeration. Only devices whose
driver with DM_FLAG_PRE_RELOC set will be bound. Any other generic
devices except bridges won't be bound.

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