Bring over the initial, CPU-only UART support for the AR933x SoC.

This implements the kernel glue needed (getc, putc, rxready).

This isn't a 16550 UART, even if the datasheet overview claims so.

The Linux ar933x support was used as a reference, however the uart code
is a reimplementation.

Attentive viewers will note that the uart code is based off of the ns8250
code and the UART bus code is a stubbed-out version of this.  I'll be
replacing it with non-stubbed versions soon, making this a fully featured
driver.

Tested:

* AP121 reference board (AR933x), booting through the mountroot> prompt;
  then doing some basic interactive tests in ddb.
Tie in the AR933x support into -HEAD.
Remove dependency code that caused a double inclusion.

Pointed out by: rpaulo and others
Revert previous change in favor of a fix to the actual dtrace
libraries that addresses a #pragma issue.