powerpc: switch to GENERIC_PCI_IOMAP

powerpc copied pci_iomap from generic code, probably to avoid
pulling the rest of iomap.c in.  Since that's in
a separate file now, we can reuse the common implementation.

The only difference is handling of nocache flag,
that turns out to be done correctly by the
generic code since arch/powerpc/include/asm/io.h
defines ioremap_nocache same as ioremap.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>