arm-dt: Enable DT proc updates.

This simple patch enables dynamic changes of the DT tree on runtime
to be visible to the device-tree proc interface.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>