Initialize amlogic RTC based on SoC instead of DTS

[john-feith.com]

Summary

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.

Test Plan

Tested on YYHD18 (aml8726-m3), VSATV102 (aml8726-m6), and
ODROIDC1 (aml8726-m8b).

[ganbold]

sys/arm/amlogic/aml8726/aml8726_rtc.c

396

Why would add here break?

[john-feith.com]

I've made the suggested change (as well as propagating the change to other files which I've piggybacked onto this review).

[john-feith.com]

I've responded to the inline comments.