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.

True for the power usage, but I would argue the other way around: Enable the external crystal per default and run at full speed (as this is what we do for all our boards at the moment). Let's then rather put in some optional configurations that let the board run on lower frequencies if really needed.