drivers/rtc/rtc-sirfsoc.c: add alarm_irq_enable support

Add missed alarm_irq_enable() callback for CSR SiRFSoC RTCs.

[akpm@linux-foundation.org: remove unneeded cast]
Signed-off-by: hao liu <hao.liu@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>