pwm: pca9685: Support hardware readout

Implement .get_state to read-out the current hardware state.

The hardware readout may return slightly different values than those
that were set in apply due to the limited range of possible prescale and
counter register values.

Also note that although the datasheet mentions 200 Hz as default
frequency when using the internal 25 MHz oscillator, the calculated
period from the default prescaler register setting of 30 is 5079040ns.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>