drivers/rtc/rtc-ab8500.c: ensure correct probing of the AB8500 RTC when Device Tree is enabled

Without this patch, if Device Tree is enabled the AB8500 RTC wouldn't get
probed at all, as there is no reference to it from platform code.  This
patch ensures the driver is probed during normal DT start-up.

[akpm@linux-foundation.org: checkpatch fixes]
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>