ARM: dts: meson8b: add the RMII pins

Some boards use an RMII Ethernet PHY which requires fewer pins than the
RGMII PHYs. Add a separate eth_rmii_pins node which does not include the
pins which are only required for RGMII (but not for RMII) PHYs.

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