powerpc/watchdog: Use hrtimers for per-CPU heartbeat

Using a jiffies timer creates a dependency on the tick_do_timer_cpu
incrementing jiffies. If that CPU has locked up and jiffies is not
incrementing, the watchdog heartbeat timer for all CPUs stops and
creates false positives and confusing warnings on local CPUs, and
also causes the SMP detector to stop, so the root cause is never
detected.

Fix this by using hrtimer based timers for the watchdog heartbeat,
like the generic kernel hardlockup detector.

Cc: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Reported-by: Ravikumar Bangoria <ravi.bangoria@in.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Reported-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>