ARM64: dts: meson-axg: add ethernet mac controller

Add DT info for the stmmac ethernet MAC which found in
the Amlogic's Meson-AXG SoC, also describe the ethernet
pinctrl & clock information here.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>