net: stmmac: update the module description of the dwmac-meson driver

The dwmac-meson glue driver supports Meson6 and Meson8 SoCs. Newer SoCs
are supported by the dwmac-meson8b driver.

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