ARM: imx: Add doc/imx/ to i.MX MAINTAINERS entry

Make sure i.MX maintainers are CCed on doc/imx/ patches.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 6d02176..8e7a8dd 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -306,6 +306,7 @@
 F:	board/freescale/*mx*/
 F:	board/freescale/common/
 F:	common/spl/spl_imx_container.c
+F:	doc/imx/
 F:	drivers/serial/serial_mxc.c
 F:	include/imx_container.h