rockchip: spl: rk3399: disable DDR security regions for SPL

The RK3399 hangs during DMA of the Designware MMC controller, when
performing DMA-based transactions in SPL due to the DDR security settings
left behind by the BootROM (i.e. accesses to the first MB of DRAM are
restricted... however, the DMA is likely to target this first MB, as it
transfers from/to the stack).

System security is not affected, as the final security configuration is
performed by the ATF, which is executed after the SPL stage.

With this fix in place, we can now drop 'fifo-mode' in the DTS for the
RK3399-Q7 (Puma).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>