MFC r323032, r323053, r323058, r323059, r323084, r323114, r323127:
Add NTB driver for PLX/Avago/Broadcom PCIe switches.

This driver supports both NTB-to-NTB and NTB-to-Root Port modes (though
the second with predictable complications on hot-plug and reboot events).
I tested it with PEX 8717 and PEX 8733 chips, but expect it should work
with many other compatible ones too. It supports up to two NT bridges
per chip, each of which can have up to 2 64-bit or 4 32-bit memory windows,
6 or 12 scratchpad registers and 16 doorbells. There are also 4 DMA engines
in those chips, but they are not yet supported.

While there, rename Intel NTB driver from generic ntb_hw(4) to more specific
ntb_hw_intel(4), so now it is on par with this new ntb_hw_plx(4) driver and
alike to Linux naming.

MFC r303494: Once more refactor KPI between ntb_transport(4) and if_ntb(4).

New design allows to attach multiple consumers to ntb_transport(4) instance.
Previous design obtained from Linux theoretically allowed that, but was not
practically usable (Linux also has only one consumer driver now).

MFC r302520: Replace NTB man page with more detailed and up to date.

Sponsored by: iXsystems, Inc.