Enable compilation of Marvell XHCI driver in arm64 GENERIC

Enabled driver can be used on boards equipped with Marvell Armada
3700/7k/8k SoCs.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12253