armv8/layerscape: Update MMU table with execute-never bits

For most device addresses excution shouldn't be allowed. Revise
the MMU table to enforce execute-never bits. OCRAM, DDR and IFC
are allowed for excution.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Alison Wang <alison.wang@freescale.com>
Reported-by: Zhichun Hua <zhichun.hua@freescale.com>