lm4f: Implement GPIO configuration and control

Everything needed to set up and control the GPIO pins is implemented,
EXCEPT setting up interrupts. This is the subject of a subsequent patch.

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

Implement an API to specifiy the interrupt trigger for GPIO pins, and
control interrupts. This completes the GPIO API.

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