MFC r354702:
Make ntb(4) send bus_get_dma_tag() requests to parent buses passing real
bus' child pointers instead of grandchilds.

DMAR does not like requests from devices not parented directly by PCI.

MFC r323126: Make NTB drivers report more info via NewBus methods.

MFC r303429, r303437:
Once more refactor KPI between NTB hardware and consumers.

New design allows hardware resources to be split between several consumers.
For example, one BAR can be dedicated for remote memory access, while other
resources can be used for packet transport for virtual Ethernet interface.
And even without resource split, this code allows to specify which consumer
driver should attach the hardware.

From some points this makes the code even closer to Linux one, even though
Linux does not provide the described flexibility.

MFC r302484: NewBus'ify NTB subsystem.

This follows NTB subsystem modularization in Linux, tuning it to FreeBSD
native NewBus interfaces. This change allows to support different types
of hardware with different drivers, support multiple NTB instances in a
system, ntb_transport module use for needs other then if_ntb, etc.

Sponsored by: iXsystems, Inc.