dt-bindings: google,cros-ec-pwm: Add the new -type compatible

Update google,cros-ec-pwm node documentation to mention the
google,cros-ec-pwm-type compatible as a valid alternative.

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