MIPS: IP27: Remove pfn_t.

In the Linux kernel traditionally pfns are represented by an unsigned long.
However a few bits of the SGI IP27 platform code that were ported from
IRIX are using pfn_t for historic reasons.  This is conflicting with
KVM's use of pfn_t.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>