Enable MBUS bridge configuration in mv_rtc driver

This patch fixes sporadic problems with updating time
with mv_rtc driver by configuring access to it via MBUS.
For this purpose already existing second set of resources
in rtc@3800 node of Armada 38x DT is used.

Submitted by: Dominik Ermel <der@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D10901