lm4f: Add UART "convenience" macros

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