ram: rockchip: add common code for sdram driver

There are some function like os_reg setting, capacity detect functions,
can be used as common code for different Rockchip SoCs, add a
sdram_common.c for all these functions.

Signed-off-by: YouMin Chen <cym@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/drivers/ram/rockchip/Kconfig b/drivers/ram/rockchip/Kconfig
index 4f274e0..dcc06b3 100644
--- a/drivers/ram/rockchip/Kconfig
+++ b/drivers/ram/rockchip/Kconfig
@@ -5,6 +5,12 @@
 	help
 	  This enables support for ram drivers Rockchip SoCs.
 
+config ROCKCHIP_SDRAM_COMMON
+	bool "Enable rockchip sdram common driver"
+	depends on TPL_RAM || SPL_RAM
+	help
+	  This enable sdram common driver
+
 if RAM_ROCKCHIP
 
 config RAM_ROCKCHIP_DEBUG