powerpc/mpc85xx: SECURE BOOT - Bypass PAMU in case of secure boot

By default, PAMU's (IOMMU) are enabled in case of secure boot.
Disable/bypass them once the control reaches the bootloader.

For non-secure boot, PAMU's are already bypassed in the default
SoC configuration.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>