drivers: pwm_nrfx: Improve tracking of PWM peripheral usage

This is a follow-up to commit 63d6cfd6.

Use a bit mask to store information about channels that need to be
driven by the PWM peripheral instead of inspecting the `seq_values`
array each time.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>