Added kconfig for MXC_UART driver.

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: NJagan Teki <jagan@amarulasolutions.com>