da9063_rtc: Add new driver for the Dialog Semiconductor DA9063 RTC

This is a simple RTC present in the PMIC, supporting 1s precision.

This is the PMIC on SiFive's HiFive Unmatched.

Reviewed by: mhorne, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36200