dt-bindings: leds: leds-mt6323: Document mt6331 compatible

Add mediatek,mt6331-led compatible for the LED controller found
in the MT6331 PMIC.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230601110813.2373764-2-angelogioacchino.delregno@collabora.com