powerpc/board: SPL: Enable malloc flag in global data.

For malloc to work in SPL framework enable GD_FLG_FULL_MALLOC_INIT
flag in global data after allocating memory using mem_malloc_init.

Signed-off-by: Sumit Garg <sumit.garg@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>