arm64: dts: mediatek: mt8183: add keyboard node

MT8183 has an on-SoC keyboard controller commonly used for volume
up/down buttons.

List it in the SoC dts so that boards can enable/use it.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220720-mt8183-keypad-v2-6-6d42c357cb76@baylibre.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>