dt-bindings: pwm: mxs-pwm: Increase #pwm-cells

We need to increase the pwm-cells for the optional flags parameter, in
order to implement support for polarity setting via DT.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>