net: stmmac: dwmac-meson8b: Add support for the Meson8m2 SoC

The Meson8m2 SoC uses a similar (potentially even identical) register
layout as the Meson8b and GXBB SoCs for the dwmac glue.
Add a new compatible string and update the module description to
indicate support for these SoCs.

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