MIPS: Add set/clear CP0 macros for PageGrain register

Build set and clear macros for the PageGrain register.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9289/


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