dt-bindings: phy-mtk-tphy: add two optional properties for u2phy

Add two optional properties, one for tuning J-K voltage by INTR,
another for disconnect threshold, both of them are related with
connect detection
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/phy-mtk-tphy.txt

@@ -52,6 +52,8 @@ Optional properties (PHY_TYPE_USB2 port (child) node):
 - mediatek,eye-vrt	: u32, the selection of VRT reference voltage
 - mediatek,eye-term	: u32, the selection of HS_TX TERM reference voltage
 - mediatek,bc12	: bool, enable BC12 of u2phy if support it
+- mediatek,discth	: u32, the selection of disconnect threshold
+- mediatek,intr	: u32, the selection of internal R (resistance)
 
 Example: