rockchip: rk3328: set DDR as non-secure in SPL

Set DDR as non-secure so that MMC DMA can access.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
[cherry picked from https://github.com/rockchip-linux/u-boot/commit/bfe741ab9eb4f97371a4e6c24185419d57a3a75f and https://github.com/rockchip-linux/u-boot/commit/73d952acc8cc1ddad6652ba71895d9fe928c1e4b

 with minor modifications]
Signed-off-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>