MIPS: CM: Add GCR_L2_CONFIG register accessors

Provide accessor functions for the GCR_L2_CONFIG register introduced
with CM3, and define the bits included in the register.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10639/


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