rockchip; rk3399: disable SRAM security region

Some host like SD and eMMC may use DMA to transter data to SRAM,
set memory to non-secure to make sure the address can be accessed.

The security of SRAM in OS suppose to initialized in ATF bl31, and
the SPL is before the bl31.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>