uboot-phytec

binman: Convert GetFdtSet() to use a dict

At present this function returns a set of device-tree filenames. It has no
way of returning the actual device-tree object. Change it to a dictionary
so that we can add this feature in a future patch.

Also drop fdt_set since it is no-longer used.

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