binman: Add special subnodes to the nodes generated by split-elf

binman: Add special subnodes to the nodes generated by split-elf

Special nodes, hash and signature, is not being added to the nodes
generated for each segment in split-elf operation.

Copy the subnode logic used in _gen_fdt_nodes to _gen_split_elf to
ensure special nodes are added to the generated nodes.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>