Add support for AHCI on ARMADA38X

This file provides support for AHCI mode on Armada38x
and adds new optional AHCI device to arm/mv/files.mv.

Submitted by: Konrad Adamczyk <ka@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: zbb
Differential revision: https://reviews.freebsd.org/D9222