dt-bindings: Add mfd/cros_ec definitions

Add a dt-bindings include file for cros_ec devicetree definition, define
a pair of special purpose PWM channels in it.

Signed-off-by: Fabio Baltieri <fabiobaltieri@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>