net: Kconfig: Add FEC_MXC entry

Added kconfig for FEC_MXC driver.

Cc: Joe Hershberger <joe.hershberger@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>