frv: switch to GENERIC_PCI_IOMAP

frv uses a version of pci_iomap that simply
casts and returns back the start address.
Looking closely, both ioremap and ioport_map seem to
do this on this platform, so the generic pci_iomap
will DTRT automatically.

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