libfdt: Add fdt_check_full() function

This new function implements a complete and thorough check of an fdt blob's
structure.  Given a buffer containing an fdt, it should return 0 only if
the fdt within is structurally sound in all regards.  It doesn't check
anything about the blob's contents (i.e. the actual values of the nodes and
properties), of course.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Simon Glass <sjg@chromium.org>