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

Add some optional properties for eye diagram test and BC12 of
u2phy

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>