Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add some initial board support for the AR934x and the Qualcomm Atheros
DB120 development board.

The AR934x SoCs are a MIPS74k based system with increased RAM addressing
space, some scratch-pad RAM, an improved gige switch PHY and 2x2 or 3x3
on-board dual-band wifi.

This support isn't complete by any stretch; it's just enough to bring
the board up for others to tinker with. Notably, the MIPS74k support
is broken. However it boots enough to echo some basic probe/attach
messages, before dying somewhere in the TLB code.

Thankyou to Qualcomm Atheros for their continued support of me doing
open source work with their hardware.

Tested:

* AR9344, mips74k