This change has the following effects:
- Reduces the resolution of the RTC sub-second counter from 30.52us to
122.07us.
- Allows RTC.calibration() to now support positive values (as well as
negative values).
- Reduces VBAT current consumption in standby mode by a small amount.
For general purpose use 122us resolution of the sub-second counter is
good enough, and the benefits of full range calibration and minor reduction
in VBAT consumption are worth the change.
