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.

At startup the CPU is initialized with the 8Mhz source clock prescaled to 1Mhz, we then change that prescaler to 0 so it runs at the full 8Mhz (cpu.c/clk_init)

There is a possibility of using a 48Mhz clock as source but that would use more power.