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>