rtc: add ability to push out an existing wakealarm using sysfs

This adds the ability for the rtc sysfs code to handle += characters at
the beginning of a wakealarm setting string.  This will allow the user
to attempt to push out an existing wakealarm by a provided amount.

In the case that the += characters are provided but the alarm is not
active -EINVAL is returned.

his is useful, at least for my purposes in suspend/resume testing.  The
basic test goes something like:

1. Set a wake alarm from userspace 5 seconds in the future

2. Start the suspend process (echo mem > /sys/power/state)

3. After ~2.5 seconds if userspace is still running (using another
   thread to check this), move the wake alarm 5 more seconds

If the "move" involves an unset of the wakealarm then there's a period
   of time where the system is midway through suspending but has no wake
   alarm.  It will get stuck.

We'd rather not remove the "move" since the idea is to avoid a cancelled
suspend when the alarm fires _during_ suspend.  It is difficult for the
test to tell the difference between a suspend that was cancelled because
the alarm fired too early and a suspend that was

Signed-off-by: Bernie Thompson <bhthompson@chromium.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Doug Anderson <dianders@chromium.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>