lm4f: Switch GPIOs to using AHB bus ***WARNING***

The AHB bus allows faster control of GPIO pins versus the older APB bus.
The GPIO ports A through H default on the APB bus. Change the GPIOx
defines in gpio.h to use the base address of the AHB bus.

There's another reason to use the AHB bus: ports K tand highercan only be
accessed via the AHB bus.

***WARNING***
To work, GPIO acces to the AHB bus must be explictly enabled via the
GPIOHBCTL register. Without any additional change, this patch will break
any code using the older APB bus. If the GPIOHBCTL is not properly
modified, any acces to the GPIO register will cause a hard fault.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: "Pretty-ize" the look of Doxygen comments in gpio.c/.h

Put the Doxygen blocks in well-formatted C comments. This looks better
and is less distracting than putting them in unformatted comments.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add all GPIO ports to memorymap.h

The base addresses for GPIO ports J through Q were missing. Add them.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add GPIO register definitions

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