powerpc/watchdog: Moderate touch_nmi_watchdog overhead

Some code can go into a tight loop calling touch_nmi_watchdog (e.g.,
stop_machine CPU hotplug code). This can cause contention on watchdog
locks particularly if all CPUs with watchdog enabled are spinning in
the loops.

Avoid this storm of activity by running the watchdog timer callback
from this path if we have exceeded the timer period since it was last
run.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>