rockchip: sdram_common: add common dram_init_banksize

dram_init_banksize() can be common used by all SoCs, move it into
sdram_common.c

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/rk3128/rk3128.c b/arch/arm/mach-rockchip/rk3128/rk3128.c
index 11bba14..ee176de 100644
--- a/arch/arm/mach-rockchip/rk3128/rk3128.c
+++ b/arch/arm/mach-rockchip/rk3128/rk3128.c
@@ -2,6 +2,9 @@
 /*
  * Copyright (c) 2017 Rockchip Electronics Co., Ltd
  */
+#include <common.h>
+
+DECLARE_GLOBAL_DATA_PTR;
 
 int arch_cpu_init(void)
 {