drivers: pwm: nrfx: adjust PWM driver to fast PWM120

Fast PWM120 instance works with 320MHz clock, thus
pwm_nrfx_get_cycles_per_sec needs to be adjusted,
applying correct clock frequency.
Also, it uses cachable RAM, thus sys_cache function
needs to be added to flush the cached sequence.

Signed-off-by: Michał Stasiak <michal.stasiak@nordicsemi.no>