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.
