alarmtimer: implement minimum alarm interval for allowing suspend

alarmtimer suspend return -EBUSY if the next alarm will fire in less
than 2 seconds.  This allows one RTC seconds tick to occur subsequent
to this check before the alarm wakeup time is set, ensuring the wakeup
time is still in the future (assuming the RTC does not tick one more
second prior to setting the alarm).  If suspend is rejected, hold a
wakeup source for 2 seconds to process the alarm prior to reattempting
suspend.

Change-Id: If38e2568da0ea01dfee6e00323ce7e2c00f2f110
Signed-off-by: Todd Poynor <toddpoynor@google.com>