rtc: ds1307 make it possible to share an irq

It's possible to have RTC irq shared with other device (e.g.
mpc8349e-mitx board shares ds1339 irq with phy one).  Handle this in
driver.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>