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 wanted to convey to the user that these were not arbitrary values but part of an exhaustive list.

Like if you see `xtal_freq = 16_000_000;` you might think that you can change this value to any other value (within reason) but that's not the case here and I thought `xtal   = "X16_0MHz";` conveyed that more clearly. It also saves me having a lookup table to convert the frequency to the crystal ID stored in the register. I agree that the syntax is not great though...