samd21: map 4 hwtimer channels onto 2 periph timers

Multiplex 4 hwtimer channels onto 2 TC timers.
I don't like this, the `hwtimer_arch` stuff should only care about the low-level driver interface, and I shouldn't contain any hw-specific references.

I suggest the following approach: have two callbacks:
```c
void irq_handler_0(int channel) {
    timeout_handler((short)channel);
}
```
and 
```c
void irq_handler_1(int channel) {
    timeout_handler((short)(channel + 2));
} 
```
This the hwtimer_arch abstraction does not need to care about specific registers.