Add TI Tiva C controller support.

I put everything in one massive commit even though there are some minor modification to generic code (notably in platform tree).

Currently supported:
 - GPIO
 - System clock configuration
 - UART TX with polling
 - Timer support with polling
 - Platform tree support
The naming format looks a bit strange, but I didn't read the reference manual yet. Does `_` mean `.`? Could it be more simple to just use integer constants measured in hz?