powerpc/mm: Use PAGE_FACTOR

PAGE_FACTOR was defined to reflect the difference between configured
page size and fixed 4KB page size. Replace (PAGE_SHIFT - HW_PAGE_SHIFT)
with PAGE_FACTOR.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>