MIPS: CPS: Introduce register modify (set/clear/change) accessors

MIPS: CPS: Introduce register modify (set/clear/change) accessors

For read-write registers introduce accessor functions that simplify the
task of modifying a subset of bits within the register. set_* functions
set bits to 1, clear_* functions clear bits to 0 & change_* functions
set bits specified in a mask to an arbitrary value.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17004/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>