dt-bindings: phy-mtk-tphy: add properties for U2 slew rate calibrate

Add two properties of ref_clk and coefficient used by U2 slew rate
calibrate which may vary on different SoCs

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>