dt-bindings: pwm: mediatek,pwm-disp: Document power-domains property

Allow the power-domains property to the PWM_DISP block as on some SoCs
this does need at most one power domain.

Fixes: b09b179b

 ("dt-bindings: pwm: Convert pwm-mtk-disp.txt to mediatek,pwm-disp.yaml format")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240404081808.92199-1-angelogioacchino.delregno@collabora.com
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>