powerpc/mpc8xxx: SECURE BOOT- Disable law 0 for non PBL platforms

ISBC creates a LAW 0 entry for non PBL platforms, which is not
disabled before transferring the control to uboot.
The LAW 0 entry has to be disabled.

Signed-off-by: Aneesh Bansal <aneesh.bansal@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>