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 AR724X chipset support.

This is untested but should at least allow an AR724X to boot.

The current code is lacking the detail needed to expose the PCIe bus.
It is also lacking any NIC, PLL or flush/WB code.