PageGrain needs rw bitmask which differs between MIPS architectures.
In pre-R6 if RIXI is supported, PageGrain.XIE and PageGrain.RIE are writeable,
whereas in R6 they are read-only 1.

On MIPS64 mtc0 instruction left shifts bits 31:30 for MIPS32 backward
compatiblity, therefore there are separate mtc0 and dmtc0 helpers.
Signed-off-by: NLeon Alrae <leon.alrae@imgtec.com>
Reviewed-by: NYongbok Kim <yongbok.kim@imgtec.com>