tools: k3_fit_atf: add DM binary to the FIT image

Add DM (device manager) firmware image to the fit image that is loaded by
R5 SPL. This is needed with the HSM rearch where the firmware allocation
has been changed slightly.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <kristo@kernel.org>