dts/bindings/rtc/rtc-device.yaml: alarms-count optional

This commit makes the alarms-count dts property of the
rtc-device.yaml optional, setting the default to 0.

This simplifies the dts rtc dts node by not requiring
the property to be set to 0 if it is not supported.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>