MIPS: Move definition of SMP processor id register to header file

The definition of the CP0 register used to save the smp processor
id is repicated in many files, move them all to thread_info.h.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5708/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>