freescale: Create the board/freescale/common/imx/ directory

freescale: Create the board/freescale/common/imx/ directory

In order to have a better hierarchy for the genimage scripts used by
NXP mx25, mx51, mx53, mx6, mx7 SoCs, let's place them inside the
board/freescale/common/imx/ directory.

This helps in creating a more natural separation between the mxs scripts
that are placed inside the board/freescale/common/mxs/ directory.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>