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
I don't really like implicit defaults, this is worth documenting in function doc, or, better be configurable (so drop a `TODO` in here).