rockchip: ringneck: fix SDRAM init fail

CONFIG_RAM_PX30_DDR4 got renamed to CONFIG_RAM_ROCKCHIP_DDR4 in commit
26f92be0 ("ram: rockchip: Add common ddr type configs"). Since both
patchsets were merged unbeknownst to the other, the conflict wasn't
detected while testing each patchset individually and could only be
observed after a merge to master branch.

Fixes: c925be73 ("rockchip: add support for PX30 Ringneck SoM on Haikou Devkit")
Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>