arm64: dts: renesas: eagle: add EtherAVB support

Define the Eagle board  dependent part of the EtherAVB device node.
Enable DHCP  and NFS root for the kernel booting.

Based  on the original (and large) patch by Vladimir Barinov.
Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/r8a77970-eagle.dts

@@ -18,10 +18,11 @@ / {
 
 	aliases {
 		serial0 = &scif0;
+		ethernet0 = &avb;
 	};
 
 	chosen {
-		bootargs = "ignore_loglevel";
+		bootargs = "ignore_loglevel rw root=/dev/nfs ip=dhcp";
 		stdout-path = "serial0:115200n8";
 	};
 
@@ -43,3 +44,14 @@ &extalr_clk {
 &scif0 {
 	status = "okay";
 };
+
+&avb {
+	renesas,no-ether-link;
+	phy-handle = <&phy0>;
+	status = "okay";
+
+	phy0: ethernet-phy@0 {
+		rxc-skew-ps = <1500>;
+		reg = <0>;
+	};
+};