ab8500-chargalg: Use hrtimer

Timers used for charging safety and maintenance must work even when
CPU power has collapsed. By using hrtimers with the realtime clock
the system is able to trigger an alarm that wakes-up the CPU and makes
it possible to handle events.

Allow a little slack time of 5 minutes for the hrtimers to allow CPU
to be woken-up in a more optimal power saving way. A 5 minute delay
to time-out timers relative to hours does not impact on safety.

Signed-off-by: Lee Jones <lee.jones@linaro.org>