mpc5121: determine RAM size using get_ram_size()

Configure CONFIG_SYS_MAX_RAM_SIZE address range in
DDR Local Access Window and determine the RAM size.
Fix DDR LAW afterwards using detected RAM size.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
diff --git a/include/configs/mpc5121ads.h b/include/configs/mpc5121ads.h
index 8ecc9e1..5281042 100644
--- a/include/configs/mpc5121ads.h
+++ b/include/configs/mpc5121ads.h
@@ -85,6 +85,7 @@
 #endif
 #define CONFIG_SYS_DDR_BASE		0x00000000	/* DDR is system memory*/
 #define CONFIG_SYS_SDRAM_BASE		CONFIG_SYS_DDR_BASE
+#define CONFIG_SYS_MAX_RAM_SIZE		0x20000000
 
 #define CONFIG_SYS_IOCTRL_MUX_DDR	0x00000036