ARM64: dts: meson-axg: add RMII pins for ethernet controller

Comparing to RGMII interface, the RMII interface require few pins.
So it's worth describing them here.

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