AR933x CPU device improvements:

* Add baud rate and divisor programming code. See below for more
  information.

* Flesh out ar933x_init() to disable interrupts and program the initial
  console setup.

* Remove #if 0'ed code from ar933x_term().

* Explain what these functions do.

Now, the baud rate and divisor code comes from Linux, as a submission
to the OpenWRT project and Linux kernel from
Gabor Juhos <juhosg@openwrt.org>.

The original ticket for this code is https://dev.openwrt.org/ticket/12031 .

I've contacted Gabor and asked for his permission to also licence the patch
in question (which covers this code) to BSD lience and he's agreed.
Hence why I'm including it here in FreeBSD.

Tested:

* AP121 (AR9330)