specification: rework 'core' chapters

Rework the chapters covering CPU clusters, indirect buses, and
execution domains to be more formal and to use the same style as the
base specification's bindings chapter.

- Add explicit definitions for the values of properties within each
  binding as much as possible, in the same style as the DT spec itself.
  Where this is not possible, leave some FIXME comments, in many cases
  with links to associated GitHub issues.

- Rework examples to try to build concepts more progressively, while
  still standing on their own in each case. Where this is not
  possible due to something else being a bit unclear, leave FIXMEs.

- Consolidate the implicit flags chapter into the execution domains
  chapter. It seems to make better sense in this page, since it is
  describing properties in the openamp,domain-v1 binding.

- Fix stray occurrences of "openamp,domain" by replacing with
  "openamp,domain-v1"

- Move some YAML out of the specification/source directory, since
  domains-yaml.md is not part of the RST spec at present

Due to the need to cross-reference other chapters, it wasn't possible
to do this in smaller, one-per-chapter commits, so one big commit it
is.

Fixes: devicetree-org#129
Fixes: devicetree-org#133

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

specification/domains-yaml.md

@@ -99,6 +99,21 @@ sram flags definitions and usage:
 
 ~~~
 
+Implict Flags Example
+---------------------
+
+The Implicit Flags Properties in the system devicetree specification
+can also be defined in YAML. For example:
+
+~~~
+  access-implicit-default:
+    secure: true
+    allow-secure: true
+    requested: true
+    coherent: false
+    virtualized: true
+    qos: 99
+~~~
 
 Bus Firewalls
 -------------

specification/source/chapter1-introduction.rst

@@ -62,6 +62,8 @@ specification:
    node. This allocation may differ across designs based on the SoC, or
    between boots on the same design.
 
+.. _usage-environments:
+
 Usage Environments
 ------------------