rockchip: sdram: Ensure ram_base is correct in SPL

rockchip: sdram: Ensure ram_base is correct in SPL

Most Rockchip SoCs use 0x0 as DRAM base address, however some SoCs use
0x60000000 and RK3576 use 0x40000000 as DRAM base address.

CFG_SYS_SDRAM_BASE is defined with correct address for each SoC and
U-Boot proper use this to set correct gd->ram_base in setup_dest_addr().

SPL never assign any value to gd->ram_base and instead use the default,
0x0. Set correct gd->ram_base in dram_init() to ensure its correctness
in SPL.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>