board: ti: j72xx: README: update build instructions and image formats

Update build instructions and image formats based on HSM rearch. A new
DM image is added into the build, which gets executed right after R5
SPL finishes its job.

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