arm/km: fix u-boot update functionality

Due to the new I2C framework we need to adapt the u-boot update
function. Due to the new framework all i2c leafs behind a mux are
present in the system and not only those who are defined and used. So it
is bus number 5 after the rework.

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