k3-*-binman: dts: Pack u-boot.dtb instead of soc specific dtb

FIT signature requires the updates to u-boot.dtb and the DTB that we
pack don't get updates with the changes of the signature node.

Pack u-boot.dtb as the default DTB so that the signature node changes
can be reflected in them.

(Note, this is only packaging the primary platform and the secondary
 platform will require manual changes for the FIT signature enablement)

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
[ add additional boards that were missing ]
Signed-off-by: Kamlesh Gurudasani <kamlesh@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>