Add LM4F UART implementation

[RFC]
Let's consider this a draft, and think on it for a day or two before we merge it. Comments and suggestions are very welcome.

Don't forget to check the documentation.

The examples that come along with this are here:
https://github.com/mrnuke/libopencm3-examples/tree/lm4f_uart
Those will have their own merge request later on/