arm64: dts: meson: add dwmac-3.70a to ethmac compatible list

After discussing with Amlogic, the Synopsys GMAC version used by
the gx and axg family is the 3.70a. Set this is in DT

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>