lm4f: Implement UART Rx and Tx interrupt control

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add basic UART echo example

Nothing fancy, just a demonstration of the blocking send and
recieve.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Modify uart example to use new clock API

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