drivers: pwm_nrfx: Add support for PWM_POLARITY_INVERTED flag

Add support for inverting of PWM channel outputs in the pwm_nrfx driver
by properly handling the `PWM_POLARITY_INVERTED` flag.
The dts properties that were used so far for inverting of the outputs
("nordic,invert" and "chX-inverted") are kept as they are needed for
setting of the initial polarity, i.e. for setting the inactive state
of the outputs before any PWM signal generation is requested for them.

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