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
Well that code is a mess because I'm trying to handle a range of very different configs. If you run directly from an oscillator without a PLL you can't use the divisor, so in that case specifying a divisor should be an error. So instead of having very complicated tests in the PT I just decided to ignore errors for now.

I think most users will want to run from the external oscillator with the PLL enabled and `div = 5` to get a sysclk frequency of 80MHz and that code supports that.

I think to handle the clock tree correctly my code should be split in several functions to avoid relying on `Options<>`, boolean flags and dirty stuff like that. I decided not to bother with that for the moment because I think it's "good enough" for the time being. Also, I cannot really implement a complete clock support as long as I don't know how we handle low-power modes.

If we want to make the PT syntax identical across implementations maybe we should take a higher level approach there? Let the user specify the clock source and desired output frequency and let the platform-specific code decide the rest of the configuration. Now that only works if we're sure we'll be able to generate the right configuration in all cases.