arm: add support for the suen3 board from keymile

Add support for the ARM part of the mgcoge2, named suen3.
This board is based on the Marvell Kirkwood (88F6281) SoC.
As there come more board variants, common config options
are collected in include/configs/km_arm.h. Also, this board
use common code for all keymile boards, which is stored in
board/keymile/common/common.c

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Heiko Schocher <hs@denx.de>