rtc: imxdi: add wakeup support

The DryIce-based RTC supports alarms that trigger an interrupt.

Configure this interrupt as a wakeup source that wakes the system up
from standby mode.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210511161244.16111-1-martin@kaiser.cx