rockchip: sdram: change sdram_init return value to int

Change-Id: Iccd78d83e898683d7315dfa1670a0308a5863824
Signed-off-by: YouMin Chen <cym@rock-chips.com>

rockchip: add rk3036 sdram driver

add rk3036 sdram driver so we can set up sdram in SPL

Signed-off-by: Lin Huang <hl@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>