rtc: tw4030 add alarm/update interfaces

- implement alarm_irq_enable
- return correct error code when registering fails

[dbrownell@users.sourceforge.net: build fixes, force 1/sec irqs]
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Samuel Ortiz <sameo@openedhand.com>
Cc: rtc-linux@googlegroups.com
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>