ARM: dts: Use the PWM polarity flags

Replace the numerical polarity flags with the PWM_POLARITY_INVERTED
symbolic constant.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>