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: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>