ntb: Add Intel Xeon Gen4 support

The NTB hardware of XEON Ice lake and Sapphire Rapids has register mapping changes
Add a new NTB_XEON_GEN4 device type and use it to conditionalize driver logic differs

Reviewed by: vangyzen, dab
Sponsored by: Dell Technologies
Differential Revision: https://reviews.freebsd.org/D43291