lm4f: Add doxygen documentation for the USB driver

Add minimal documentation for the USB driver. The internal functions and
structs are excluded.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
lm4f: Add functions for controlling USB interrupts

Add functions to enable and disable USB interrupts, and document how to
use these functions to run usbd_poll() from the usb ISR.

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