ARM: dts: meson: use the real ethernet clock on Meson8 and Meson8b

Until now clk81 was used as gate clock for the ethernet controller on
Meson8 whereas Meson8b did not configure a gate clock at all. Use
CLKID_ETH for both SoCs, which is the real gate clock for the ethernet
controller.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>