dt-bindings: mfd: Add compatible for MT6331 PMIC

Add a compatible for the MT6331 PMIC MFD device.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220520124039.228314-5-angelogioacchino.delregno@collabora.com