MIPS: Add CPC probe, access functions

This patch introduces code to probe for a MIPS Cluster Power Controller
& accessor functions to allow for easy register access. This support
code will be used by a subsequent patch.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6361/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>