drivers/rtc/rtc-ds3232.c: make it possible to share an irq

It's possible to have RTC irq shared with other device (e.g.  t4240qds
board shares ds3232irq with phy one).  Handle this in driver.

Signed-off-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>