Add support for the Texas Instruments CC2538 ARM Cortex-M3 MCU and developer kit.

Could be simplified to 
```C
case TIMER_0:
    NVIC_EnableIRQ(TIMER_0_IRQn_1);
    NVIC_EnableIRQ(TIMER_0_IRQn_2);
    break;
```
with `TIMER_0_IRQn_1` and `TIMER_0_IRQn_2` defined in `periph_conf.h`.