Introduce support for Epson RX-8803 RTC.

This patch introduces support for Epson RX-8803 RTC controller accessible
over I2C bus. It has a resolution of 1 sec.
Support for interrupt based alarm was not implemented.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D24364