In a couple of places sam460ex_load_device_tree() calls "raw" libfdt
functions which can fail, but doesn't check for error codes. At best,
if these fail the guest will be silently started in a non-standard state,
or it could fail entirely.
Fix this by using the _FDT() helper macro which aborts on a libfdt failure.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>