ram: rk3399: migrate to use common code

For there are some structures and functions are common for all rockchip SoCs,
migrate to use the common code so that we can clean up reduandent codes.

Signed-off-by: YouMin Chen <cym@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/Kconfig b/arch/arm/mach-rockchip/Kconfig
index 5f1ad51..7746b66 100644
--- a/arch/arm/mach-rockchip/Kconfig
+++ b/arch/arm/mach-rockchip/Kconfig
@@ -184,6 +184,7 @@
 	select DM_REGULATOR_FIXED
 	select BOARD_LATE_INIT
 	imply ROCKCHIP_COMMON_BOARD
+	imply ROCKCHIP_SDRAM_COMMON
 	imply SPL_ROCKCHIP_COMMON_BOARD
 	imply TPL_SERIAL_SUPPORT
 	imply TPL_LIBCOMMON_SUPPORT