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
Yeah, by "hack" I meant that this code will have to move a lot when we transition to an interrupt driven system with a scheduler (that's the plan, right?) so I didn't bother making it very clean for now as I expect that I'll have plenty of opportunities to revisit timer declaration and initialization.