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.

In general our timers run at 1MHz, that was not feasible when using 16 bit timers. I'd propose the following changes
hwtimer_cpu.h: (2 timers remains, maps to hardware channels)
```diff
+#define HWTIMER_MAXTIMERS   2               /**< the CPU implementation supports 2 HW timers */
-#define HWTIMER_SPEED       (124999U)       /**< the HW timer runs with 125kHz */
+#define HWTIMER_SPEED       (1000000U)       /**< the HW timer runs with 1MHz */
```
timer.c (for `TIMER_0`):
```diff
-        /* sourced by 8MHz with Presc 64 results in 125kHz clk */
-        TIMER_0_DEV.CTRLA.bit.PRESCALER = TC_CTRLA_PRESCALER_DIV64_Val;
+        /* sourced by 8MHz with Presc 8 results in 1MHz clk */
+        TIMER_0_DEV.CTRLA.bit.PRESCALER = TC_CTRLA_PRESCALER_DIV8_Val;
```