net: dt-bindings: Document the new Meson8b and GXBB DWMAC bindings

This patch adds the documentation for the DWMAC ethernet controller
found in Amlogic Meson 8b (S805) and GXBB (S905) SoCs.
The main difference between the Meson6 glue is that different registers
(with different layout) are used.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>