MIPS: Fix execution hazard during watchpoint register probe

Writing a value to a WatchLo* register creates an execution hazard, so
if its value is then read before that hazard is cleared then said value
may be invalid. The mips_probe_watch_registers function must therefore
clear the execution hazard between setting the match bits in a WatchLo*
register & reading the register back in order to check which are set.

This fixes intermittent incorrect watchpoint register probing on some
MIPS cores such as interAptiv & proAptiv.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/5474/


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