MAINTAINERS: Add missing directory

arch/arm/mach-imx/ is part of the i.MX support. Add it.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

MAINTAINERS

@@ -789,6 +789,7 @@ L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
 T:	git git://git.pengutronix.de/git/imx/linux-2.6.git
 F:	arch/arm/mach-mx*/
+F:	arch/arm/mach-imx/
 F:	arch/arm/plat-mxc/
 
 ARM/FREESCALE IMX51