MAINTAINERS: imx: Add an entry for the serial driver

Currently, when running ./scripts/get_maintainer.pl on serial_mxc.c
no i.MX maintainer is returned.

Fix it by adding an entry for this driver.

Reported-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Peng Fan <peng.fan@nxp.com>