soc/arm: Increase nRF timer default to the cycle rate.

The nRF timer runs at only 32 kHz, so there's little reason to try to
divide it to get a synthesized tick rate.  Just use the raw clock as
the tick rate, which provides maximal precision and very
singnificantly simplifies the generated code for the ISR.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>