powerpc: ARCH_PFN_OFFSET should be unsigned long

pfns are unsigned long, but MEMORY_START is phys_addr_t.  This leads
to page_to_pfn() returning phys_addr_t, and thus type mismatches in a few
print statements.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>