dt-bindings: phy-mtk-tphy: add a new reference clock

Usually the digital and analog phys use the same reference clock,
but on some platforms, they are separated, so add another optional
clock to support it.
In order to keep the clock names consistent with PHY IP's, use
the da_ref for analog phy and ref clock for digital phy.

Signed-off-by: Chunfeng Yun <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

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

···
 - 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
+		  "ref": 48M reference clock for HighSpeed (digital) phy; and 26M
+			reference clock for SuperSpeed (digital) phy, sometimes is
 			24M, 25M or 27M, depended on platform.
+		  "da_ref": the reference clock of analog phy, used if the clocks
+			of analog and digital phys are separated, otherwise uses
+			"ref" clock only if needed.
 
 - mediatek,eye-src	: u32, the value of slew rate calibrate
 - mediatek,eye-vrt	: u32, the selection of VRT reference voltage