arm/km: fix i2c mux define for km_kirkwood_128m16 target

Due to the i2c mux rework in u-boot we now have only to specify the
busnumber and not the whole mux configuration.

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