ARM: kirkwood: convert the mv88f6281gtw_ge board to DT

This commit converts the mv88f6281gtw_ge Kirkwood board to use a
Device Tree representation, except for the Ethernet devices and the
DSA switch. Even though the mv643xx_eth driver has a DT binding,
converting this board to use it is for now left on the side because it
doesn't use a simple PHY, but a DSA switch instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>