binman: Drop unnecessary field in output_fdt_info

At present we store an entry as the third field in output_fdt_info[].
This is only used to get the type of the entry. Of course multiple entries
may have this same type. Also the entry type is the key to this dict, so
we can use that instead.

Drop the field and update GetUpdateNodes() to suit. Improve the comment for
output_fdt_info a little while here.

Signed-off-by: Simon Glass <sjg@chromium.org>