if_ntb: Add Xeon link watchdog register writes

This feature is disabled by default. To enable it, tune
hw.if_ntb.enable_xeon_watchdog to non-zero.

If enabled, writes an unused NTB register every second to demonstrate to
a hardware watchdog that the NTB device is still alive. Most machines
with NTB will not need this -- you know who you are.

Sponsored by: EMC / Isilon Storage Division