NTB Tool: Test driver for NTB hardware drivers.

NTB Tool driver is meant for testing NTB hardware driver functionalities,
such as doorbell interrupts, link events, scratchpad registers and memory
windows. This is a port of ntb_tool driver from Linux. It has been
verified on top of AMD and PLX NTB HW drivers.

Submitted by: Arpan Palit <arpan.palit@amd.com>
Cleaned up by: mav
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D18819