imx: move ATF to the back of the FIT to fix loading over yModem

With yModem the FIT Image is only supplied once, so we can only
seek forward in the yModem supplied image and never backwards.
With the recent changes to the SPL mechanism, including loading
U-Boot first, FDT after, then the loadables, we must also reorder
the FIT image script to make sure that the loadables are last in
the FIT image.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Tested-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>