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

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

Add in the vendor / device id so the ath_ahb glue works.

Now that interrupt driven uart IO is working, flip this back on.

Implement AR933x polled IO uart bus code.

This implements the bus transmit/receive/sigchg/ipend methods with
a polled interrupt handler (ipend) rather than enabling hardware
interrupts.

The FIFO is faked at 16 bytes deep for now, just so the transmit
IO side doesn't suck too bad (the callout frequency limits how quickly
IO is flushed to the sender, rather than scheduling the callout more
frequently whilst there's active TX. But I digress.)

Tested:

* Atheros AP121 (AR9330) reference board, booting to multi-user interactive
mode.

Initial (unfinished!) AR933x support.