MIPS: CM: use __raw_ memory access functions

The CM registers use native endianness, so using plain readl & writel
will produce incorrect results on big endian systems.

Reported-by: Jeffrey Deans <jeffrey.deans@imgtec.com>
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6656/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>