dt-bindings: Remove more cases of 'allOf' containing a '$ref'

Another round of 'allOf' removals that came in this cycle.

json-schema versions draft7 and earlier have a weird behavior in that
any keywords combined with a '$ref' are ignored (silently). The correct
form was to put a '$ref' under an 'allOf'. This behavior is now changed
in the 2019-09 json-schema spec and '$ref' can be mixed with other
keywords. The json-schema library doesn't yet support this, but the
tooling now does a fixup for this and either way works.

This has been a constant source of review comments, so let's change this
treewide so everyone copies the simpler syntax.

Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/tegra/nvidia,tegra20-pmc.yaml

@@ -85,9 +85,8 @@ properties:
       CPU power good signal from external PMIC to PMC is enabled.
 
   nvidia,suspend-mode:
-    allOf:
-      - $ref: /schemas/types.yaml#/definitions/uint32
-      - enum: [0, 1, 2]
+    $ref: /schemas/types.yaml#/definitions/uint32
+    enum: [0, 1, 2]
     description:
       The suspend mode that the platform should use.
       Mode 0 is for LP0, CPU + Core voltage off and DRAM in self-refresh