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