arm64: dts: meson-gx: Fix Ethernet MAC address unit name

Unit names should use hyphens instead of underscores to not cause
warnings.

Fixes: bfe59f9 ("ARM64: dts: amlogic: gxbb: Enable NVMEM")
Suggested-by: Vyacheslav Bocharov <adeep@lexina.in>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230111211350.1461860-5-martin.blumenstingl@googlemail.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>

arch/arm64/boot/dts/amlogic/meson-gx.dtsi

@@ -233,7 +233,7 @@
 			reg = <0x14 0x10>;
 		};
 
-		eth_mac: eth_mac@34 {
+		eth_mac: eth-mac@34 {
 			reg = <0x34 0x10>;
 		};