Add reset capability to mv_rtc driver

This commit enables optional reset of the RTC, in case
its registers' contents did not sustain the reboot or power-off/on
sequence. Without it, further usage of RTC is impossible
(e.g. writing values to RTC_TIME register will not succeed).

The reset is performed only if Clock Correction register
does not comprise RTC_NOMINAL_TIMING, what helps to distinguish,
whether the software configured RTC before or it comprises
the default value.

Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D10900