rockchip: rk3188: sdram: Set correct sdram base

Right now we're setting the wrong value of 0 as base in the ram_info struct,
which is obviously wrong for the rk3188. So instead set the correct value
we already have in CONFIG_SYS_SDRAM_BASE.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Simon Glass <sjg@chromium.org>