MIPS: pgtable-bits: Move the CCA bits out of the core's ifdef blocks

Define all the CCA bits outside the ifdef blocks for specific cores
but also allow cores to override them if necessary.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7400/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>