ia64/PCI: Use ioremap() instead of open-coded equivalent

Depositing __IA64_UNCACHED_OFFSET in the upper address bits is essentially
equivalent to ioremap(): it converts a CPU physical address to a virtual
address using the ia64 uncacheable identity map.

Call ioremap() instead of doing the phys-to-virt conversion manually with
__IA64_UNCACHED_OFFSET.

Note that this makes it obvious that (a) we're putting a virtual address in
a struct resource, and (b) we're passing a virtual address to ioremap()
below in the PCI_ROM_RESOURCE case.  These are both pre-existing problems
that I'll resolve next.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>