dt-bindings: phy-mtk-tphy: make the ref clock optional

Make the ref clock optional, then we no need refer to a fixed-clock
in DTS anymore when the clock of USB3 PHY comes from oscillator
directly
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>

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

@@ -34,12 +34,6 @@ Optional properties (controller (parent) node):
 
 Required properties (port (child) node):
 - reg		: address and length of the register set for the port.
-- clocks	: a list of phandle + clock-specifier pairs, one for each
-		  entry in clock-names
-- clock-names	: must contain
-		  "ref": 48M reference clock for HighSpeed analog phy; and 26M
-			reference clock for SuperSpeed analog phy, sometimes is
-			24M, 25M or 27M, depended on platform.
 - #phy-cells	: should be 1 (See second example)
 		  cell after port phandle is phy type from:
 			- PHY_TYPE_USB2
@@ -48,6 +42,13 @@ Required properties (port (child) node):
 			- PHY_TYPE_SATA
 
 Optional properties (PHY_TYPE_USB2 port (child) node):
+- clocks	: a list of phandle + clock-specifier pairs, one for each
+		  entry in clock-names
+- clock-names	: may contain
+		  "ref": 48M reference clock for HighSpeed anolog phy; and 26M
+			reference clock for SuperSpeed anolog phy, sometimes is
+			24M, 25M or 27M, depended on platform.
+
 - mediatek,eye-src	: u32, the value of slew rate calibrate
 - mediatek,eye-vrt	: u32, the selection of VRT reference voltage
 - mediatek,eye-term	: u32, the selection of HS_TX TERM reference voltage