lm4f: Add basic UART implementation

Add basic functionality needed to setup the UART and send/recieve data.
Interrupts, DMA, and more advanced features are not implemented in this patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Implement UART interrupt control

Add a complete API for controlling the UART interrupts.
Doxygen documentation with inline code examples is also provided in this patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>