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
not exactly. Timers are explicit as they can do lots of other things apart from downcounting ticks. What is going to go away is `prescale`-style things, those should be specified in time units based on clock configuration.