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>

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

@@ -27,6 +27,10 @@ Optional properties (controller (parent) node):
  - reg		: offset and length of register shared by multiple ports,
 		  exclude port's private register. It is needed on mt2701
 		  and mt8173, but not on mt2712.
+ - mediatek,src-ref-clk-mhz	: frequency of reference clock for slew rate
+		  calibrate
+ - mediatek,src-coef	: coefficient for slew rate calibrate, depends on
+		  SoC process
 
 Required properties (port (child) node):
 - reg		: address and length of the register set for the port.