MIPS: Use C0_KScratch (if present) to hold PGD pointer.

Decide at runtime to use either Context or KScratch to hold the PGD
pointer.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1876/


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