fdt_region: Check for a single root node of the correct name

At present fdt_find_regions() assumes that the FIT is a valid devicetree.
If the FIT has two root nodes this is currently not detected in this
function, nor does libfdt's fdt_check_full() notice. Also it is possible
for the root node to have a name even though it should not.

Add checks for these and return -FDT_ERR_BADSTRUCTURE if a problem is
detected.

CVE-2021-27097

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Bruce Monroe <bruce.monroe@intel.com>
Reported-by: Arie Haenel <arie.haenel@intel.com>
Reported-by: Julien Lenoir <julien.lenoir@intel.com>