atrtc: export function to set RTC

This is going to be used by the Xen clock on Dom0 in order to set the RTC of
the host. The current logic in atrtc_settime is moved to atrtc_set and the
unused device_t parameter is removed from the atrtc_set function call so it
can be safely used by other callers.

Sponsored by: Citrix Systems R&D
Reviewed by: kib, jhb
Differential revision: https://reviews.freebsd.org/D6067