ibm,opal/diagnostics device tree entries

The diagnostics node under ibm,opal describes a userspace-to-firmware interface, supporting the runtime processor recovery diagnostics functions.

Note: Some systemd init scripts look for the presence of the path /ibm,opal/diagnostics in order to run the opal-prd daemon.

The properties of a prd node are:

/ {
   ibm,opal {
     diagnostics {
       compatible = "ibm,opal-prd";
   };
  };
};