Separate out PCI attachment from the main AHCI driver. Move checks of
PCI IDs into quirks, which mostly fit (though you'd get no argument
from me that AHCI_Q_SATA1_UNIT0 is oddly specific). Set these quirks
in the PCI attachment. Make some shared functions public so that PCI
and possibly other bus attachments can use them.

The split isn't perfect yet, but it is functional. The split will be
perfected as other bus attachments for AHCI are written.

Sponsored by: Netflix
Reviewed by: kan, mav
Differential Revision: https://reviews.freebsd.org/D699