Add missed alarm_irq_enable() callback for CSR SiRFSoC RTCs.

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