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.comSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Documentation/devicetree/bindings/pwm/mediatek,pwm-disp.yaml

@@ -52,6 +52,9 @@ properties:
       - const: main
       - const: mm
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg