pwm: sun4i: Drop legacy callbacks

Remove the legacy callbacks .enable(), .disable(), .set_polarity() and
.config().

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-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>