arm64: dts: renesas: Add compatible properties to AR8031 Ethernet PHYs

Add compatible values to Ethernet PHY subnodes representing Atheros
AR8031 PHYs on RZ/G2 boards.  This allows software to identify the PHY
model at any time, regardless of the state of the PHY reset line.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/3f1b58756f149f0c634c66abaecc88e699f4c3cc.1631174218.git.geert+renesas@glider.be

arch/arm64/boot/dts/renesas/beacon-renesom-som.dtsi

@@ -59,6 +59,8 @@ &avb {
 	status = "okay";
 
 	phy0: ethernet-phy@0 {
+		compatible = "ethernet-phy-id004d.d074",
+			     "ethernet-phy-ieee802.3-c22";
 		reg = <0>;
 		interrupt-parent = <&gpio2>;
 		interrupts = <11 IRQ_TYPE_LEVEL_LOW>;