libfdt: Check for multiple/invalid root nodes

It is possible to construct a devicetree blob with multiple root nodes.
Update fdt_check_full() to check for this, along with a root node with an
invalid name.

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>