rtcwake: add S5 support

rtcwake: add S5 support

Based on patch from
   Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>
at
   https://bugzilla.redhat.com/show_bug.cgi?id=449115

Piergiorgio's note about S5:
> According to the ACPI specifications, chapter 4.7.2.4 "Real Time
> Clock Alarm", the wakeup from RTC, when supported, should work from
> *sleep* state S1-S3 and, optionally, from S4.
>
> Note 3 (same chapter) says that S5 is *not* a sleep state and should
> not be supported. Actually it also says that: "The OS will disable
> the RTC_EN bit prior to entering the G2/S5 or G3 states regardless."
>
> Nevertheless, on all PC supporting the RTC wakeup I tested, all were
> able to wake from S5.

Signed-off-by: Karel Zak <kzak@redhat.com>