imx: spl: fix imx8m secure boot

cherry-picked from NXP code:
719d665a87c6: ("MLK-20467 imx8m: Fix issue for booting signed image through uuu")

which fixes secure boot on imx8m based boards. Problem was
that FIT header and so IVT header too, was loaded to
memallocated address. So the ivt header address coded
in IVT itself does not fit with the real position.

Signed-off-by: Heiko Schocher <hs@denx.de>
Tested-by: Tim Harvey <tharvey@gateworks.com>