atrtc: expose power loss as sysctl

Exposing the a power loss of the rtc as an sysctl makes it easier to
detect an empty cmos battery.

Reviewed by: manu
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38325

(cherry picked from commit 55f1ca209d37d7b5a6faf57b23c7341cb7f84ee6)