dt-bindings: phy: mediatek: tphy: add a property for power-domains

Include the MediaTek TPHY in a power domain.  Add the 'power-domains'
property in the Device Tree (DT) Schema and set 'maxItems: 1'.

Also, include a reminder in the description of the 'power-domains'
property.

Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241012035016.17667-1-macpaul.lin@mediatek.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: macpaul-lin-mtk

Documentation/devicetree/bindings/phy/mediatek,tphy.yaml

@@ -125,6 +125,16 @@ properties:
     $ref: /schemas/types.yaml#/definitions/uint32
     default: 28
 
+  power-domains:
+    description:
+      The TPHY of MediaTek should exist within a power domain. The
+      developer should be aware that the hardware design of MediaTek TPHY
+      does not require the addition of MTCMOS. If the power to the TPHY
+      is turned off, it will impact other functions. From the current
+      perspective of USB hardware design, even if MTCMOS is added to the
+      TPHY, it should remain always on.
+    maxItems: 1
+
 # Required child node:
 patternProperties:
   "^(usb|pcie|sata)-phy@[0-9a-f]+$":