Add the reference clock for each supported chip.

Obtained from:	Linux (openwrt)
Commit initial (unfinished!) support for the AR933x series of embedded
CPUs.

The AR933x is a mips24k based SoC with an AR9380 series SoC on board,
two gigabit ethernet interfaces and an internal 10/100mbit ethernet
switch.  There's also the normal interfaces (USB, ethernet, uart, GPIO.)

The downside? There's a non-ns8250 UART device.

With a very basic UART driver (not in this commit) the SoC is initialised
and boots up.  I'll commit the UART code soon and then link it into the
general setup path.

This code is a re-implementation based from the Linux kernel / openwrt
AR933x support.

TODO:

* UART (obviously)
* All of the ethernet, USB and wifi SoC glue, including ethernet PLL
  programming.