phy: phy-mtk-tphy: add configurable parameters for slew rate calibrate

There are two parameters, ref_clk and coefficient, for U2 slew rate
calibrate which may vary on different SoCs, here allow them to be
configurable

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>