i2c, multibus: get rid of CONFIG_I2C_MUX

CONFIG_I2C_MUX is replaced through the new i2c multibus/multiadapter
framework, configured through CONFIG_SYS_I2C. As CONFIG_I2C_MUX
is only used on the keymile boards, and they are now completely
moved to the new framework, remove CONFIG_I2C_MUX.

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