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
(cherry picked from commit 129028c79c6c287c3c028714e3e7993299357b6c)