83xx, i2c: add mux support for fsl_i2c

This patch adds I2C mux support for the fsl_i2c driver. This
allows you to add "new" i2c busses, which are reached over
i2c muxes. For more infos, please look in the README and
search for CONFIG_I2C_MUX.

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>