xilinx: zynqmp: Generate different u-boot.itb for MULTI_DTB_FIT

When MULTI_DTB_FIT is enabled fit-dtb.blob fit image is created which
contain all DTBs listed by CONFIG_OF_LIST. And with DTB_RELESELECT there is
a need to handle it as one file with DTBs in it not as separate DTBs in
u-boot.its/itb.
That's why extend mkimage_fit_atf.sh to generate u-boot.itb correctly.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>