dt-bindings: net: phy: mscc: vsc8531: remove compatible from required properties

Compatible isn't a required property for PHYs so let's remove it from
the binding DT of the VSC8531 PHYs.
Acked-by: NRob Herring <robh@kernel.org>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NQuentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mscc-phy-vsc8531.txt

 * Microsemi - vsc8531 Giga bit ethernet phy
 
-Required properties:
-- compatible	: Should contain phy id as "ethernet-phy-idAAAA.BBBB"
-		  The PHY device uses the binding described in
-		  Documentation/devicetree/bindings/net/phy.txt
-
 Optional properties:
 - vsc8531,vddmac	: The vddmac in mV. Allowed values is listed
 			  in the first row of Table 1 (below).