powerpc/p1010rdb: SECURE BOOT enabled for NAND

In case of secure boot from NAND, the DDR is initialized by the
BootROM using the config words (CF_WORDS) in the CF_HEADER
and u-boot image is copied from NAND to DDR by the BootROM.
So, CONFIG_SYS_RAMBOOT has been defined for Secure Boot from NAND.

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