sam460ex: Make sam460ex_load_device_tree() handle all errors internally

sam460ex_load_device_tree() handles nearly all possible errors by simply
exiting (within helper functions and macros).  It handles two early error
cases by returning an error.

There's no particular point to this, so make it handle those directly as
well, removing the need for the caller to handle a failure.  As a bonus it
gives us more specific error messages.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>