rtc: bbnsm: Add the bbnsm rtc support

The BBNSM module includes a real time counter with alarm.
Add a RTC driver for this function.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20230215024117.3357341-3-ping.bai@nxp.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>