arm64: dts: renesas: r8a77970: Set EtherAVB phy mode to "rgmii"

Set the "phy-mode" property of EtherAVB device to "rgmii" and let board
files override it if the installed PHY layer provides delays for the
RX/TX channels.
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/r8a77970.dtsi

@@ -519,7 +519,7 @@ avb: ethernet@e6800000 {
 			clocks = <&cpg CPG_MOD 812>;
 			power-domains = <&sysc R8A77970_PD_ALWAYS_ON>;
 			resets = <&cpg 812>;
-			phy-mode = "rgmii-id";
+			phy-mode = "rgmii";
 			iommus = <&ipmmu_rt 3>;
 			#address-cells = <1>;
 			#size-cells = <0>;