mips: Fix CPC_BASE_ADDR mask to match datasheet

mips: Fix CPC_BASE_ADDR mask to match datasheet

According to 'MIPS32® interAptivTM Multiprocessing
System Programmer’s Guide' CPC_BASE_ADDR takes bits [31:15].

This change is tested ith mt7621 which wasn't working without it.

Signed-off-by: Nikolay Martynov <mar.kolya@gmail.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11766/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>