MIPS: BMIPS: Add SMP support code for BMIPS43xx/BMIPS5000

Initial commit of BMIPS SMP support code.  Smoke-tested on a variety of
BMIPS4350, BMIPS4380, and BMIPS5000 platforms.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2977/


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