lm4f-doc: Sanitize doxygen structure

By somo copypaste error, the doxygen group rcc_defines was defined twice.
Don't do that. Keep Ken's doxygen structure and drop the old one.

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

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