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>
diff --git a/MAINTAINERS b/MAINTAINERS
index 36a2b69..8334618 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -264,6 +264,7 @@
 F:	arch/arm/include/asm/arch-vf610/
 F:	arch/arm/include/asm/mach-imx/
 F:	board/freescale/*mx*/
+F:	drivers/serial/serial_mxc.c
 
 ARM HISILICON
 M:	Peter Griffin <peter.griffin@linaro.org>