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

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

[ Upstream commit a62d196e89887c029d5aef409135f9a2a8667268 ]

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

Fixes: df77f7735786 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>