msm: watchdog: store worst case watchdog slack time

Record the amount of slack the watchdog count has each time we pet.
Record this both in ticks as read from the watchdog status register, and
in nanoseconds as read from the timer. This will allow us to catch any
potential discrepancies from the expected watchdog tick rate.

Having statistics across a number of runs will allow us to see if we
commonly are close to the watchdog bark time, or if that only happens in
exceptional cases.

Change-Id: I6e6c9995d53a7093e1e9d939e9da6c0c35c3490f
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>