dt-bindings: regulator: mediatek,mt6331: Add missing compatible

dt-bindings: regulator: mediatek,mt6331: Add missing compatible

This binding had no compatible and for this reason would not be
applied to anything: add the missing "mediatek,mt6331-regulator"
comaptible.

Fixes: 6385e21692bb ("regulator: Add bindings for MT6331 regulator")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>