arm64: dts: renesas: beacon: Fix Ethernet PHY mode

While networking works fine in RGMII mode when using the Linux generic
PHY driver, it fails when using the Atheros PHY driver.
Fix this by correcting the Ethernet PHY mode to RGMII-RXID, which works
fine with both drivers.

Fixes: a5200e6 ("arm64: dts: renesas: rzg2: Convert EtherAVB to explicit delay handling")
Reported-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/2a4c15b2df23bb63f15abf9dfb88860477f4f523.1632465965.git.geert+renesas@glider.be

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

@@ -50,6 +50,7 @@
 &avb {
 	pinctrl-0 = <&avb_pins>;
 	pinctrl-names = "default";
+	phy-mode = "rgmii-rxid";
 	phy-handle = <&phy0>;
 	rx-internal-delay-ps = <1800>;
 	tx-internal-delay-ps = <2000>;