arm/km: add kmcoge5un board support

arm/km: add kmcoge5un board support

For u-boot this board is similar to mgcoge3un. But some differences
are present. We have a different SDRAM on it and therefore a new
SDRAM config file. Additionaly this board has a direct MAC/MAC
connection from the kirkwood to a marvell simple switch without a
phy inbetween, this needs a new configuration for the mvgbe driver.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Gerlando Falauto <gerlando.falauto@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>