dt-bindings: input: mtk-pmic-keys: add binding for MT6357 PMIC

Add binding documentation for the PMIC keys on MT6357.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://lore.kernel.org/r/20221005-mt6357-support-v8-1-560caaafee53@baylibre.com


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