[POWERPC] Generic OF platform driver for PCI host bridges.

When enabled in Kconfig, it will pick up any of_platform_device
matching it's match list (currently type "pci", "pcix", "pcie",
or "ht" and setup a PHB for it.

Platform must provide a ppc_md.pci_setup_phb() for it to work
(for doing the necessary initialisations specific to a given PHB
like setting up the config space ops).

It's currently only available on 64 bits as the 32 bits PCI code
can't quite cope with it in it's current form. I will fix that
later.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>