The RTC initialization values are based on the SoC which can be
determined at runtime so there's no need to set the values in
each DTS.
Tested on YYHD18 (aml8726-m3), VSATV102 (aml8726-m6), and
ODROIDC1 (aml8726-m8b).

Differential Revision: https://reviews.freebsd.org/D2588
Submitted by: John Wehle