arm: rockchip: rv1126: Set dram area unsecure for SPL

Unsecure the dram area so that MMC, USB, and SFC controllers
can able to read data from dram.

Signed-off-by: Jason Zhu <jason.zhu@rock-chips.com>
Signed-off-by: Jagan Teki <jagan@edgeble.ai>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>