F1 already has an existing RTC implementation and headers, but it is a
radically different peripheral. F2,F3,F4, F0 and L1 all have a "v2" RTC
module, using BCD time/date registers and providing alarms.
This commit provides a "dispatch" header for the appropriate family,
as well as providing most of the register bit definitions.
(Calibration and sub second registers have been skipped for now)
No examples yet for L1 or F4.