Introduce UART driver module for Armada 3700

This patch adds support for UART in Armada 3700 family.
It exposes both low-level UART interface, as well as
standard driver methods.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12250