MIPS: Get rid of hard-coded values for Malta PIIX4 fixups

Make the code more readable by using defines.

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6031/


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