Add support for Armada 3700 in the NETA driver

This patch enables using NETA driver on Marvell Armada 3700 SoC
by introducing new compatible string, modifying clock source
obtaining and also excluding unnecessary parts.
The driver is added as a build option for arm64 platforms as well.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12258