MIPS: R3000: Replace magic numbers with macros

Also include asm/mmu_context.h for ASID_MASK.

Signed-off-by: Isamu Mogi <isamu@leafytree.jp>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8291/


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