Add Marvell RTC driver to arm64 GENERIC config

Marvell Armada 80x0/70x0 SoC family uses same RTC IP as
Armada 38x. This patch adds necessary files and enable driver in
GENERIC config.

Submitted by: Rafal Kozik <rk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12200