dt-bindings: pinctrl: mediatek: use - instead of _ in examples

It should be good that no use "_" is in examples. Consequently,
those nodes in certain files which have an inappropriate name containing
"_" are all being replaced with "-".

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-gpio@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>