pwm: rockchip: Allow polarity invert on rk3288

The rk3288 has the ability to invert the polarity of the PWM.  Let's
enable that ability.  Note that this increases pwm_cells to 3 for
rk3288.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Caesar Wang <caesar.wang@rock-chips.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>