dt-bindings: mfd: syscon: Disallow simple-bus with syscon

"syscon" is a system controller with registers having their own
functions, thus not really a trivial MMIO simple bus.  "simple-bus" on
the other hand is just a bus on which multiple devices sit and the
"simple" means no functions are allowed here.

Combination of both "syscon" and "simple-bus" is abuse of DT for easier
instantiating of Linux device drivers so add a schema to disallow that.

Unfortunately there are a few old cases of that patterns, so add
exceptions:

1. "cznic,turris1x-cpld" and "img,pistachio-cr-periph" are already used
   in upstream DTS.

2. TI has several DTSI with a child of SCM device (e.g. "ti,am3-scm")
   using "syscon" and "simple-bus" but without a dedicated compatible
   documented anywhere.  Add new compatibles for such cases.

Additionally, add comments around code enforcing two or three
compatibles: it is similar safeguard detecting incorrect bindings.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260608-n-dt-bindings-simple-bus-syscon-v3-1-4eba9ec1212a@oss.qualcomm.com
Signed-off-by: Lee Jones <lee@kernel.org>

Documentation/devicetree/bindings/mfd/syscon-common.yaml

@@ -32,6 +32,7 @@ properties:
   compatible:
     contains:
       const: syscon
+    # Always require a specific compatible for syscon
     minItems: 2
     maxItems: 5  # Should be enough
 
@@ -52,11 +53,44 @@ allOf:
           contains:
             const: simple-mfd
     then:
+      # Always require a specific compatible for syscon with simple-mfd
       properties:
         compatible:
           minItems: 3
           maxItems: 5
 
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: simple-bus
+    then:
+      # simple-bus conflicts with syscon - if a device is a system controller
+      # with miscellaneous registers, then it has at least one dedicated
+      # function thus it is not a simple bus. Allow existing exceptions.
+      if:
+        properties:
+          compatible:
+            not:
+              contains:
+                # This list CANNOT grow
+                enum:
+                  - cznic,turris1x-cpld
+                  - img,pistachio-cr-periph
+                  - ti,am3352-scm-conf
+                  - ti,am4372-scm-conf
+                  - ti,dm814-scm-conf
+                  - ti,dm8168-scm-conf
+                  - ti,dra7-scm-conf
+                  - ti,omap2-scm-conf
+                  - ti,omap3-scm-conf
+                  - ti,omap4-sysc-padconf-global
+                  - ti,omap5-scm-wkup-conf
+                  - ti,omap5-sysc-padconf-global
+      then:
+        required:
+          - incorrect-usage-of-simple-bus-and-syscon
+
 additionalProperties: true
 
 examples: