task_wdt: Add suspend and resume API functions

The goal is to be able to use the Task Watchdog on a system that is also
using power management to reach low-power modes. In some low-power modes,
the watchdog channels can't be feed anymore.

The task_wdt_suspend() function allows to prepare the Task Watchdog for
a system low-power mode, in which the hardware watchdog (if enabled) is
also suspended.

The task_wdt_resume() function will reschedule the internal timer that
manages the channels, feed all channels and also the hardware watchdog.
Thus, the application is good to go and has enough time to feed the
channels by itself.

Signed-off-by: Adrien Ricciardi <aricciardi@baylibre.com>