MIPS: Scheduler support for HARDWARE_WATCHPOINTS.

Here we hook up the scheduler.  Whenever we switch to a new process,
we check to see if the watch registers should be installed, and do it
if needed.

Signed-off-by: David Daney <ddaney@avtrex.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>