Add support for Allwinner A31 RTC controller.

Very similar to the A10 RTC controller with a few register offsets changed
and a different location for the LOSC switch status.

Submitted by: Emmanuel Vadot <manu@bidouilliste.com>
Reviewed by: jmcneill
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5500