tests: latency_measure: tweak frdm_k64f config

The FRDM K64F board runs at 120 MHz and has a 24 bit timer.
According to its timer driver, its MAX_TICKS (which comes into
play for a tickless kernel) is calculated as ...
     CYC_PER_TICK = 120 MHz / ticks per second
     MAX_TICKS = (COUNTER_MAX / CYC_PER_TICK) - 1

To achieve a minimum value of MAX_TICKS=1, ticks per second must
be set to at least 15. Experimentation has shown that setting the
number of ticks per second below this threshhold on a tickless
kerenl leads to unreliable timestamps.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>