MAINTAINERS: Fix ARM FREESCALE IMX files

- Remove arch/arm/cpu/arm926ejs/imx/ which is not available
- arch/arm/cpu/imx-common/ => arch/arm/imx-common/

Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>