rtc: pl031: avoid exposing alarm if no interrupt

If the RTC has no interrupt, there is little point in exposing the RTC
alarm capabilities, as it can't be used as a wakeup source nor can it
deliver an event to userspace.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>