MIPS: ath79: ar9331: Add OpeneEmbed SOM9331 Board

Add SOM9331 based Board. It has 3 LAN ports, usb to uart controller and
USB type A port.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>