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 think we'll have a dedicated discussion on clocking after I go through reference manual. It looks a bit confusing and I wonder if we can make it simpler, ideally all platforms should have the same set of attributes in `clock`.