MIPS: Handle huge pages with 64KB base page size.

When using sparsemem, we need to adjust some constants as the resulting
huge pages are 512MB in size.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3745/


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