Introduce a driver for NXP LS1046A SoC AHCI.

Implement support for AHCI controller found in
NXP QorIQ Layerscape SoCs.

Submitted by: Artur Rojek <ar@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D24466