Add support for Armada 3700 EHCI

This patch reuses ehci_mv driver by adding a support for the new
compatible string and adding ehci_mv.c to list of available options
for arm64 platforms.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12255