ARM: dts: amlogic: add missing phy-mode in ethmac node

The phy-node property is required, add it and fix:
ethernet@c9410000: 'phy-mode' is a required property
	from schema $id: http://devicetree.org/schemas/net/snps,dwmac.yaml#

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20240916-topic-amlogic-arm32-upstream-bindings-fixes-v2-8-0b20ed4f0571@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>

arch/arm/boot/dts/amlogic/meson8-minix-neo-x8.dts

@@ -94,4 +94,5 @@ &ethmac {
 	status = "okay";
 	pinctrl-0 = <&eth_pins>;
 	pnictrl-names = "default";
+	phy-mode = "rmii";
 };