arm/kirkwood: if CONFIG_SOFT_I2C is set don't set CONFIG_I2C_MVTWSI

Some boards e.g. keymile arm boards have CONFIG_CMD_I2C switched on
but they use soft i2c on kirkwood. So don't switch CONFIG_I2C_MVTWSI
on in this case.

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