dt-bindings: regulator: add regulator-fixed-clock binding

This adds the documentation to the compatible regulator-fixed-clock.
This binding is a special binding of regulator-fixed and adds the
ability to add a clock to regulator-fixed, so the regulator can be
enabled and disabled with that clock. If the special compatible
regulator-fixed-clock is used it is mandatory to supply a clock.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190910062103.39641-4-philippe.schenker@toradex.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/fixed-regulator.yaml

@@ -19,9 +19,19 @@ description:
 allOf:
   - $ref: "regulator.yaml#"
 
+if:
+  properties:
+    compatible:
+      contains:
+        const: regulator-fixed-clock
+  required:
+    - clocks
+
 properties:
   compatible:
-    const: regulator-fixed
+    enum:
+      - const: regulator-fixed
+      - const: regulator-fixed-clock
 
   regulator-name: true
 
@@ -29,6 +39,13 @@ properties:
     description: gpio to use for enable control
     maxItems: 1
 
+  clocks:
+    description:
+      clock to use for enable control. This binding is only available if
+      the compatible is chosen to regulator-fixed-clock. The clock binding
+      is mandatory if compatible is chosen to regulator-fixed-clock.
+    maxItems: 1
+
   startup-delay-us:
     description: startup time in microseconds
     $ref: /schemas/types.yaml#/definitions/uint32