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 agree that we should share this code, however the reason I changed it here is because the lpc syntax doesn't make a lot of sense to me.

```rust
uart@0 {
  baud_rate = 115200;
  mode = "8N1";
  // ...
}
```

It looks weird to be because baud_rate is alone as an integer while the rest of the UART config is in a custom-syntax string.

I don't understand why we need to single out the baud rate. So I'd be in favour of either putting everything in a special custom-syntax string (which is the approach I chose because that's what the linux kernel uses for instance) or just split everything:

```rust
uart@0 {
  baud_rate     = 115200;
  bits_per_word = 8;
  parity        = "none";
  stop_bits     = 1;
  // ...
}
```

It looks a bit verbose but I think we might have some sane defaults like `bits_per_word = 8` etc...