pci: add CONFIG_PCI_ENUM_ONLY for platforms that don't need PCI setup done

Introduce CONFIG_PCI_ENUM_ONLY variable for platforms that just want a
quick enumberation of the PCI devices, but don't need any setup work done.
This is very beneficial on platforms that have u-boot loaded by another
boot loader which does a more sophisticated job of setup of PCI devices
than u-boot.  That way, u-boot can just read what's there and get on
with life.  This is what SeaBIOS does.

Signed-off-by: Andrew Sharp <andywyse6@gmail.com>