dt-bindings: pinctrl: mediatek: add bindings for I2C2 and SPI2 on MT7623

Add missing pinctrl binding about I2C2 and SPI2 which would be used in
devicetree related files.

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