samr21: 32 hwtimer

The hwtimer did not work correctly on the samr21-xpro, I accidentally read that it has to use a 32 bit timer,
So I changed the timer peripheral to have 1 16-bit timer and 1 32-bit timer, and the hwtimer is now using the 32 bit timer.

I can see the value of adding some options for changing the cpu clock, the change in this pull request was more to reflect the current state. There might be a bit of hurdle when changing the cpu clock source because at the moment the 8Mhz clock feeds all the peripherals, and i'm not confident that we haven't coupled them too tightly.

Also, the 48Mhz is a completely internal oscillator.