Add initial support for the TP-Link Archer C7 v2.

Add initial support for the TP-Link Archer C7 v2.

The SoC, the flash, the ethernet ports and ethernet switch all work.
The USB works.

The 11ac PCIe NIC internally is at least seen by the PCIE RC, but
I haven't tried using it yet.  There's no driver and I haven't
yet swapped it out for a non-11ac chip.

The on-chip 2GHz wifi works, but there are some data errors that
get thrown up in STA mode when scanning.  I have a feeling I have
to finish the DDR flush code out and have it run correctly on the
shared interrupts; that'll take a bit of time to get right.

But if you're after an updated piece of hardware, the Archer C7 v2
is certainly there, and you can replace the 11ac NIC with a 3x3
Atheros PCIe device (eg AR9380, AR9390, AR9580, etc) and it'll
"just work".

Tested:

* TP-Link archer c7v2.