armv8: fsl-layerscape: Make DDR non secure in MMU tables

DDR has been set as secure in MMU tables. Non-secure master such
as SDHC DMA cannot access data correctly. Mixing secure and non-
secure MMU entries requirs the MMU tables themselves in secure
memory. This patch moves MMU tables into a secure DDR area.

Early MMU tables are changed to set DDR as non-secure. A new
table is added into final MMU tables so secure memory can have
2MB granuality.

gd->secure_ram tracks the location of this secure memory. For
ARMv8 SoCs, the RAM base is not zero and RAM is divided into several
banks. gd->secure_ram needs to be maintained before using. This
maintenance is board-specific, depending on the SoC and memory
bank of the secure memory falls into.

Signed-off-by: York Sun <yorksun@freescale.com>