nios2: trim CONFIG_SYS_MALLOC_LEN

Trim CONFIG_SYS_MALLOC_LEN size, because CONFIG_ENV_SIZE
is included to total memory allocation in common.h,
Signed-off-by: NThomas Chou <thomas@wytron.com.tw>
Reviewed-by: NChin Liang See <clsee@altera.com>

include/configs/nios2-generic.h

@@ -83,7 +83,7 @@
 #define CONFIG_SYS_MONITOR_BASE	(CONFIG_SYS_SDRAM_BASE + \
 					 CONFIG_SYS_SDRAM_SIZE - \
 					 CONFIG_SYS_MONITOR_LEN)
-#define CONFIG_SYS_MALLOC_LEN		(CONFIG_ENV_SIZE + 0x20000)
+#define CONFIG_SYS_MALLOC_LEN		0x20000
 
 /*
  * MISC