MIPS: Use PAGE_SIZE in assembly instead of _PAGE_SIZE.

Now that PAGE_SIZE is available to assembly directly, there is no need
to separately expose it as _PAGE_SIZE through asm-offsets.

In addition, remove _PAGE_SHIFT from asm-offsets, since it was never
needed, and is not used anywhere.

Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Signed-off-by: Tim Abbott <tabbott@ksplice.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>