dt-bindings: net: meson-dwmac: add support for the Meson8m2 SoC

The Meson8m2 SoC uses a similar (potentially even identical) register
layout for the dwmac glue as Meson8b and GXBB. Unfortunately there is no
documentation available.
Testing shows that both, RMII and RGMII PHYs are working if they are
configured as on Meson8b. Add a new compatible string to the
documentation so differences (if there are any) between Meson8m2 and the
other SoCs can be taken care of within the driver.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>