binman: test: Ensure all config references exist in /images node

binman: test: Ensure all config references exist in /images node

Several binman FIT test device trees reference image nodes such as atf
and uboot in their /configurations sections, but those image nodes
were not actually defined in the /images node. This mismatch can lead
to validation errors when stricter consistency checks are introduced.

This patch adds minimal definitions for atf and uboot under the
/images node in all relevant test DTS files.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>