dt-bindings: input: mediatek,pmic-keys: Add compatible for MT6331 keys

Add a compatible for the keys found on MT6331 PMIC.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20220914103021.43593-3-angelogioacchino.delregno@collabora.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>