powerpc: Make radeon 32-bit MSI quirk work on powernv

This moves the quirk itself to pci_64.c as to get built on all ppc64
platforms (the only ones with a pci_dn), factors the two implementations
of get_pdn() into a single pci_get_dn() and use the quirk to do 32-bit
MSIs on IODA based powernv platforms.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>