Add driver for NTB in AMD SoC.

Add driver for NTB in AMD SoC.

This patch is the driver for NTB hardware in AMD SoCs (ported from Linux)
and enables the NTB infrastructure like Doorbells, Scratchpads and Memory
window in AMD SoC. This driver has been validated using ntb_transport and
if_ntb driver already available in FreeBSD.

Submitted by:	Rajesh Kumar <rajesh1.kumar@amd.com>
MFC after:	1 month
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D18774