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

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

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

Fixes: bfe59f92d306 ("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>