specification: domains: make id sometimes optional

Previous use cases for execution domains generally required an id
property. However, as the specification is applied in more contexts,
it's becoming clear that it may not always be needed. In particular,
the Zephyr RTOS is not expected to have initial use cases for this
property (though it may in the future start using it).

To avoid forcing these users to maintain unique id properties that are
not needed, refine the binding so that 'id' is only needed in some
cases.

Fixes: devicetree-org#163
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Reviewed-by: Stefano Stabellini <stefano.stabellini@amd.com>

specification/source/chapter3-execution-domains.rst

@@ -104,8 +104,8 @@ Execution Domain Binding, v1
                                                                    the default value is zero.
    ``sram``                            SD    ``<prop encoded       See :numref:`domains-sram`. Specifies
                                              array>``              the MMIO SRAM assigned to the domain.
-   ``id``                              R     ``<u32>``             A 32-bit integer that uniquely
-                                                                   identifies the domain
+   ``id``                              SD    ``<u32>``             See :numref:`domains-id`. A 32-bit integer
+                                                                   that uniquely identifies the domain.
    ``os,type``                         SD    ``<string>``          See :numref:`domains-os-type`
    ``#access-implicit-default-cells``  SD    ``<u32>``             See :numref:`domains-implicit-flags`
    ``access-implicit-default``         SD    array                 See :numref:`domains-implicit-flags`
@@ -298,6 +298,24 @@ Within each triplet:
 - *flags* contains domain-specific flags. The number of cells in each flag is
   defined by the *#sram-flags-cells* property of the execution domain.
 
+.. _domains-id:
+
+id Property
+~~~~~~~~~~~
+
+This property may be used to provide a unique numeric identifier for the
+domain.
+
+Although it is optional in general, the *id* property is required if the
+*compatible* property of the domain node contains any string which matches one
+of the following patterns:
+
+- "xilinx,subsystem*"
+- "xen,domain*"
+
+For example, a domain whose *compatible* property includes
+"xilinx,subsystem-v1" must have an *id* property.
+
 .. _domains-os-type:
 
 os,type Property