spl: fit: allocate a temporary buffer to load the overlays

spl: fit: allocate a temporary buffer to load the overlays

If the node describing an overlay does not specify a load address, it will
be loaded at the address previously used.
Fixing it by allocating a temporary buffer that will be used as a
default load address. By default, the size of the buffer is 64kB which
should be plenty for most use cases.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>