arm64: dts: mediatek: mt8390-genio-700-evk: Configure touch vreg pins

Add a pinctrl configuration for the Touchscreen IC's power line
to make sure that the pin is configured as GPIO and to stop
relying on correct pin configuration from bootloader.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>