arm/km: add support for portl2 board

This adds support for the keymile Kirkwood BEC portl2 board. This board
relies on the km_arm (km_kirkwood) BEC.

The egiga driver is configured for a 100M full-duplex, A/N off connnection
to the backplane. This board has always ethernet present, because it is
connected to the marvell switch similar to mgcoge3un. The reset_phy
functionality is also the same to mgcoge3un.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Heiko Schocher <hs@denx.de>