imx53loco: Fix post-image.sh permission

Fix post-image.sh permission to avoid the following error:

>>>   Executing post-image script board/freescale/imx53loco/post-image.sh
/bin/bash: board/freescale/imx53loco/post-image.sh: Permission denied

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>