sam460ex: Check for errors from libfdt functions

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>