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 r323128: Increase negotiation polling period from 10ms to 100ms.

There is no big need to burn CPU if other side may be not there yet. For
example, the PLX hardware by default enables the NTB link up on reset, not
dependig on driver to do it. In case of Intel hardware this also reduces
race between MSI-X workaround negotiation and upper layers, using the same
scratchpad registers in different time.

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

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.