arm64: dts: mediatek: Add MT6878 pinmux macro header file

Add the required macros for the pinmux nodes of the MediaTek MT6878 SoC.

Signed-off-by: Igor Belwon <igor.belwon@mentallysanemainliners.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>