Add Armada 80x0/70x0 compatible to 38x RTC driver

Marvell Armada 80x0/70x0 SoC family uses same RTC IP as Armada 38x.
This patch adds Armada 8k compatible to Marvell RTC driver.

Submitted by: Rafal Kozik <rk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12186