powerpc/83xx: increment malloc heap size for the MPC832x MDS boards

The malloc buffer is not large enough to hold a flash sector (0x20000 bytes)
in addition to whatever else it normally holds, so double its size.  This
fixes a failure trying to save the environment:

=> save
Saving Environment to Flash...
Unable to save the rest of sector (122880)
. done
Protected 1 sectors

This problem probably surfaced from some other change that significantly
increased the normal memory usage, thereby not leaving enough room for
the saveenv command.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>

include/configs/MPC832XEMDS.h

@@ -181,7 +181,7 @@
 
 /* CONFIG_SYS_MONITOR_LEN must be a multiple of CONFIG_ENV_SECT_SIZE */
 #define CONFIG_SYS_MONITOR_LEN	(384 * 1024)	/* Reserve 384 kB for Mon */
-#define CONFIG_SYS_MALLOC_LEN	(128 * 1024)	/* Reserved for malloc */
+#define CONFIG_SYS_MALLOC_LEN	(256 * 1024)	/* Reserved for malloc */
 
 /*
  * Initial RAM Base Address Setup