pwm: sun4i: Improve hardware read out

Implement .get_state instead of only reading the polarity at probe time.
This allows to get the proper state, period and duty cycle.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>