makefile: add multi_dtb_fit dep

makefile: add multi_dtb_fit dep

With certain gcc compilers, the u-boot.itb is built immediately after dtb
generation.  If CONFIG_MULTI_DTB_FIT is used, it is possible that the
fit-dtb.blob is not finished in time.

This patch adds a necessary dependency to guarantee that the fit-dtb.blob
is built before attempting to build the u-boot.itb.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20221221075446.47141-1-neal.frager@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>