ARM: mvebu: armada-xp: Add RGMII pinctrl to Lenovo ix4-300d

Lenovo ix4-300d has two ethernet PHYs connected via RGMII. Add the
corresponding pinctrl settings.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-By: Benoit Masson <yahoo@perenite.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>