rockchip: add sdram_common for common functions

There are some functions like sdram_size_mb can be re-used for
different rockchip SoCs, just put them into common file.
Add board_get_usable_ram_top() for ram_top init base on
SDRAM_MAX_SIZE.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Added SDRAM_MAX_SIZE definition for RK3036:
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

fixup: 3036 fix for sdram_common
diff --git a/include/configs/rk3036_common.h b/include/configs/rk3036_common.h
index 836c5e3..1f6b5a1 100644
--- a/include/configs/rk3036_common.h
+++ b/include/configs/rk3036_common.h
@@ -39,6 +39,7 @@
 #define CONFIG_SYS_SDRAM_BASE		0x60000000
 #define CONFIG_NR_DRAM_BANKS		1
 #define SDRAM_BANK_SIZE			(512UL << 20UL)
+#define SDRAM_MAX_SIZE                  (CONFIG_NR_DRAM_BANKS * SDRAM_BANK_SIZE)
 
 #define CONFIG_SPI_FLASH
 #define CONFIG_SPI
diff --git a/include/configs/rk3188_common.h b/include/configs/rk3188_common.h
index a1e0eb7..7f57dd3 100644
--- a/include/configs/rk3188_common.h
+++ b/include/configs/rk3188_common.h
@@ -64,6 +64,7 @@
 #define CONFIG_SYS_SDRAM_BASE		0x60000000
 #define CONFIG_NR_DRAM_BANKS		1
 #define SDRAM_BANK_SIZE			(2UL << 30)
+#define SDRAM_MAX_SIZE			0x80000000
 
 #define CONFIG_SPI_FLASH
 #define CONFIG_SPI
diff --git a/include/configs/rk3288_common.h b/include/configs/rk3288_common.h
index ecf2675..7d6ef41 100644
--- a/include/configs/rk3288_common.h
+++ b/include/configs/rk3288_common.h
@@ -48,6 +48,7 @@
 #define CONFIG_SYS_SDRAM_BASE		0
 #define CONFIG_NR_DRAM_BANKS		1
 #define SDRAM_BANK_SIZE			(2UL << 30)
+#define SDRAM_MAX_SIZE			0xfe000000
 
 #define CONFIG_SPI_FLASH
 #define CONFIG_SPI
diff --git a/include/configs/rk3328_common.h b/include/configs/rk3328_common.h
index 5a06244..7f9f0c5 100644
--- a/include/configs/rk3328_common.h
+++ b/include/configs/rk3328_common.h
@@ -9,7 +9,6 @@
 
 #include "rockchip-common.h"
 
-#define CONFIG_NR_DRAM_BANKS		1
 #define CONFIG_ENV_SIZE			0x2000
 #define CONFIG_SYS_MAXARGS		16
 #define CONFIG_SYS_MALLOC_LEN		(32 << 20)
@@ -37,6 +36,7 @@
 #define CONFIG_SYS_MMCSD_FS_BOOT_PARTITION	1
 #define CONFIG_SYS_SDRAM_BASE		0
 #define CONFIG_NR_DRAM_BANKS		1
+#define SDRAM_MAX_SIZE			0xff000000
 
 #define CONFIG_SPI_FLASH
 #define CONFIG_SPI
diff --git a/include/configs/rk3368_common.h b/include/configs/rk3368_common.h
index 8ebf232..b0c858c 100644
--- a/include/configs/rk3368_common.h
+++ b/include/configs/rk3368_common.h
@@ -12,6 +12,8 @@
 #include <asm/arch/hardware.h>
 #include <linux/sizes.h>
 
+#define CONFIG_SYS_SDRAM_BASE		0
+#define SDRAM_MAX_SIZE			0xff000000
 #define CONFIG_NR_DRAM_BANKS		1
 #define CONFIG_SYS_MAXARGS		16
 #define CONFIG_BAUDRATE			115200
diff --git a/include/configs/rk3399_common.h b/include/configs/rk3399_common.h
index 44dad57..54ea97b 100644
--- a/include/configs/rk3399_common.h
+++ b/include/configs/rk3399_common.h
@@ -51,6 +51,7 @@
 /* FAT sd card locations. */
 #define CONFIG_SYS_MMCSD_FS_BOOT_PARTITION	1
 #define CONFIG_SYS_SDRAM_BASE		0
+#define SDRAM_MAX_SIZE			0xf8000000
 #define CONFIG_NR_DRAM_BANKS		1
 
 #define CONFIG_SF_DEFAULT_SPEED 20000000