[MIPS] Make PAGE_OFFSET aware of PHYS_OFFSET

For platforms that use PHYS_OFFSET and do not use a mapped kernel,
this patch automatically adds PHYS_OFFSET into PAGE_OFFSET.
Therefore there are no more needs for them to redefine PAGE_OFFSET.

For mapped kernel, they need to redefine PAGE_OFFSET anyways.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>