imx: Fix imx8m FIT script issue

The FIT config node has reversed ATF and u-boot: ATF is set to 'firmware' but
u-boot is set to 'loadables'.
This script can work previously because spl fit driver wrongly appends fdt to
all loadable images. With the issue fixed in commit 9d15d1d1 ("Revert
"common: spl_fit: Default to IH_OS_U_BOOT if FIT_IMAGE_TINY enabled"") the
u-boot in 'loadables' does not have fdt appended and fails to work.  So correct
the script by moving u-boot to 'firmware' and ATF to 'loadables'.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reported-by: Matt Porter <mporter@konsulko.com>
Tested-by: Matt Porter <mporter@konsulko.com>