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>
lm4f: Implement UART Rx and Tx interrupt enable/disable

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Check the clock source from before setting the baud divisor

The UART can be clocked from either the internal oscillator, or the system
clock. The baudrate divisor depends on this clock's frequency, so take that
into account.