Add driver for PCIe root complex on Annapurna Alpine platform.

The driver subclasses pci-host-generic and additionally
performs configuration of vendor-specific PCIe registers.

Obtained from: Semihalf
Submitted by: Michal Stanek <mst@semihalf.com>
Sponsored by: Annapurna Labs
Reviewed by: wma
Differential Revision: https://reviews.freebsd.org/D7571