rtc: alarm: have dummy set_power_on_alarm function

When !CONFIG_RTC_CLASS we need to have a dummy set_power_on_alarm
function. Otherwise, it is easy to have build issues.

Change-Id: I39310241cc2e59f672964c8bc0ee4c3607aedf4e
Signed-off-by: Maria Yu <aiquny@codeaurora.org>