dt-bindings: phy: mediatek,tphy: add property to set pre-emphasis

Add a property to set usb2 phy's pre-emphasis, which used to widen eye
opening and boost eye swing.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20220914060746.10004-2-chunfeng.yun@mediatek.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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

@@ -219,6 +219,16 @@ patternProperties:
         minimum: 1
         maximum: 15
 
+      mediatek,pre-emphasis:
+        description:
+          The level of pre-emphasis which used to widen the eye opening and
+          boost eye swing, the unit step is about 4.16% increment; e.g. the
+          level 1 means amplitude increases about 4.16%, the level 2 is about
+          8.3% etc. (U2 phy)
+        $ref: /schemas/types.yaml#/definitions/uint32
+        minimum: 1
+        maximum: 3
+
       mediatek,bc12:
         description:
           Specify the flag to enable BC1.2 if support it