Convert the mtk-pmic-keys to DT schema format.

The old binding was missing documentation for key press/release
interrupts, even though it was supported in hardware and driver,
so support for the same was added during the conversion.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220914103021.43593-2-angelogioacchino.delregno@collabora.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>