mv-common.h: increase malloc arena to 4MiB

This will fix the following error:

---8<---
UBIFS error (pid 0): ubifs_mount: Error reading superblock on volume 'ubi:root' errno=-12!
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: prafulla@marvell.com
Cc: dimax.main@gmail.com
Tested-by: Alex Xol <dimax.main@gmail.com>

include/configs/mv-common.h

@@ -92,7 +92,7 @@
 /*
  * Size of malloc() pool
  */
-#define CONFIG_SYS_MALLOC_LEN	(1024 * 1024) /* 1MiB for malloc() */
+#define CONFIG_SYS_MALLOC_LEN	(1024 * 1024 * 4) /* 4MiB for malloc() */
 
 /*
  * Other required minimal configurations