xen: arm: bootfdt: Avoid reading off the front of *_cells array

xen: arm: bootfdt: Avoid reading off the front of *_cells array

In device_tree_for_each_node the call to the callback was using
{address,size}_cells[depth - 1], which at depth 0 could read off the
front of the array.

We already handled this correctly in the rest of the loop so fixup
this instance as well.

Reported-by: Chris (Christopher) Brand <chris.brand@broadcom.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Chris (Christopher) Brand <chris.brand@broadcom.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>