rtc: alarm: Proper handling of spinlock

In alarm_shutdown hook, taking interruptible mutex
lock in interrupt disabled context. This patch fixes
alarm_shutdown function to call rtc_read_time and
rtc_set_alarm APIs in interrupt enabled context.

Change-Id: I9392d8908c19365993dccc22504f4b6f83ae8f72
CRs-Fixed: 619657
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>