dt-bindings: net: mediatek: add condition to property mediatek, pctl

The property "mediatek,pctl" is only required for SoCs such as MT2701 and
MT7623, so adding a few words for stating the condition.
Signed-off-by: NSean Wang <sean.wang@mediatek.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mediatek-net.txt

@@ -28,7 +28,7 @@ Required properties:
 - mediatek,sgmiisys: phandle to the syscon node that handles the SGMII setup
 	which is required for those SoCs equipped with SGMII such as MT7622 SoC.
 - mediatek,pctl: phandle to the syscon node that handles the ports slew rate
-	and driver current
+	and driver current: only for MT2701 and MT7623 SoC
 
 Optional properties:
 - interrupt-parent: Should be the phandle for the interrupt controller