imx: Add Makefile dependency for mkimage_fit_atf.sh

The mkimage_fit_atf.sh SPL FIT generator script requires
u-boot-nodtb.bin, but this was not enforced by the Makefile. This could
cause the generator script to be executed before u-boot-nodtb.bin has
been created.

Signed-off-by: Chris Spencer <christopher.spencer@sea.co.uk>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>