MIPS: ralink: add rt_sysc_m32 helper

We already have a read and write wrapper. This adds the missing mask wrapper.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/8001/


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