ARM64: dts: meson-gxbb: Add rmii pinctrl node and rename rgmii node

For boards only supporting 10/100 ethernet over a RMII PHY link, add
a separate pinctrl node. By the way, rename the existing node to rgmii
specific naming in all boards dts.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>