drivers: net: driver for MDIO muxes controlled over I2C

This driver is used for MDIO muxes driven over I2C.  This is currently
used on Freescale LS1028A QDS board, on which the physical MDIO MUX is
controlled by an on-board FPGA which in turn is configured through I2C.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>