dt-bindings: clock: mediatek: Add an extra required property to sgmiisys

add an extra required property "mediatek,physpeed" to sgmiisys to determine
link speed to match up the capability of the target PHY.
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/arm/mediatek/mediatek,sgmiisys.txt

@@ -9,6 +9,8 @@ Required Properties:
 	- "mediatek,mt7622-sgmiisys", "syscon"
 	- "mediatek,mt7629-sgmiisys", "syscon"
 - #clock-cells: Must be 1
+- mediatek,physpeed: Should be one of "auto", "1000" or "2500" to match up
+		     the capability of the target PHY.
 
 The SGMIISYS controller uses the common clk binding from
 Documentation/devicetree/bindings/clock/clock-bindings.txt