dt-bindings: mfd: mediatek: Add MT6397 Pin Controller

The MT6397 mfd includes a pin controller. Add binding
a description for it.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>