Add support to set polarity on PWM devices, allowing for inverted
duty cycles.

Also update the binding document to #pwm-cells = <3> to allow
passing the flags from devicetree.
Signed-off-by: NTony Prisk <linux@prisktech.co.nz>
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>