MIPS: CPS: Use GlobalNumber macros rather than magic numbers

We now have definitions for the GlobalNumber register in asm/mipsregs.h,
so use them in place of magic numbers in cps-vec.S.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17008/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>