ntb: Add Intel Xeon Gen3 support

The NTB hardware starting with Skylake has some changes to the register
map and the doorbell interface. Add a new NTB_XEON_GEN3 device type and
use it to conditionalize driver logic that differs from the existing
Xeon code.

Reviewed by: vangyzen
Discussed with: cem, Bret Ketchum <Bret.Ketchum@dell.com>
MFC after: 1 month
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D26683