specification: zero cpu-mask is "all the CPUs"

Extend the execution domain binding to say that a cpu-mask cell value
of 0 means that the domain runs on all of the CPUs in the cluster.

This seems like a common case, and it is a useful shorthand which may
help users, who could define a self-documenting macro like
"DOMAIN_CPU_MASK_ALL" or so for this.

Fixes: devicetree-org#166
Signed-off-by: Martí Bolívar <[email protected]>
Reviewed-by: Stefano Stabellini <[email protected]>

specification/source/chapter3-execution-domains.rst

@@ -151,6 +151,33 @@ Within the triplet:
   the domain runs on
 - *execution-level* is a cluster-specific execution level for the domain
 
+A zero value for the *cpu-mask* cell must be treated as though the bits for all
+CPUs in the node with phandle *cpu-cluster* were set. For example, with the
+following CPU cluster:
+
+.. code-block:: DTS
+
+   cpus_r5: cpus-r5 {
+           compatible = "cpus,cluster";
+
+           /* ... */
+
+           cpu@0 { /* ... */ };
+           cpu@1 { /* ... */ };
+   };
+
+This *cpus* property value:
+
+.. code-block:: none
+
+   cpus = <&cpus_r5 0x0 0x80000001>;
+
+must be treated identically to:
+
+.. code-block:: none
+
+   cpus = <&cpus_r5 0x3 0x80000001>;
+
 The execution level is the most privileged level that the domain can
 make use of. The permissible values for the *execution-level* cell in a
 *cpus* property depend on the CPU cluster hardware. The following