rtc: rtc-twl: Switch to using threaded irq

The driver is accessing to i2c bus in interrupt handler.
Therefore, it should use threaded irq.

Signed-off-by: Ilkka Koskinen <ilkka.koskinen@nokia.com>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>