rtc: pl031: use RTC_FEATURE_ALARM

Clear RTC_FEATURE_ALARM instead of setting set_alarm, read_alarm and
alarm_irq_enable to NULL.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210110231752.1418816-3-alexandre.belloni@bootlin.com