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

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

Implement some initial AR934x support routines.

This code reads the PLL configuration registers and correctly programs
things so the UART and such can come up.

There's MIPS74k platform issues that need fixing; but this at least brings
things up enough to echo stuff out the serial port and allow for interactive
debugging with ddb.

Tested:

* AR71xx SoCs
* AR933x SoC
* AR9344 board (DB120)

Obtained from: Qualcomm Atheros; Linux/OpenWRT