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>