dt-bindings: pwm: fix microchip corePWM's pwm-cells

corePWM is capable of inverted operation but the binding requires
\#pwm-cells of 2. Expand the binding to support setting the polarity.

Fixes: df77f773 ("dt-bindings: pwm: add microchip corepwm binding")
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>