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
while I totally see the reason in naming it `w0`, it couldn't figure it out myself without a comment. Still, adding another node in the path to resolve as `timer::wide::0` looks a bit messy. Will get back to it a bit later.