arm: zynq : Revert TZ_DDR_RAM to secure.

TZ_DDR_RAM on reset is in secure mode.
Since uboot and linux runs in full
TZ privilege secure mode, no need
to set DDR trustzone to non-secure.

Signed-off-by: Radhey Shyam Pandey <radheys@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>