mn10300: switch to GENERIC_PCI_IOMAP

The pci_iomap variant that arch/mn10300/unit-asb2305/pci-iomap.c
uses differs from the generic one in that it does
not use ioremap_nocache for PCI addresses.
However, it turns out that PCI addresses are
automatically noncached, so switching to ioremap_nocache
and to the generic implementation is safe.

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