rtc: introduce features bitfield

Introduce a bitfield to allow the drivers to announce the available
features for an RTC.

The main use case would be to better handle alarms, that could be present
or not or have a minute resolution or may need a correct week day to be set.

Use the newly introduced RTC_FEATURE_ALARM bit to then test whether alarms
are available instead of relying on the presence of ops->set_alarm.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210110231752.1418816-2-alexandre.belloni@bootlin.com