dt-bindings: mmc: sdhci-msm: Fix 'operating-points-v2 was unexpected' issue

As Rob reported in [1], there is one more issue present
in the 'sdhci-msm' dt-binding which shows up when a fix for
'unevaluatedProperties' handling is applied:

 Documentation/devicetree/bindings/mmc/sdhci-msm.example.dtb:
  mmc@8804000: Unevaluated properties are not allowed
   ('operating-points-v2' was unexpected)

Fix the same.

[1]. https://lore.kernel.org/lkml/20220514220116.1008254-1-bhupesh.sharma@linaro.org/

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220725202709.2861789-1-bhupesh.sharma@linaro.org

Documentation/devicetree/bindings/mmc/sdhci-msm.yaml

@@ -137,6 +137,8 @@ properties:
 
   max-frequency: true
 
+  operating-points-v2: true
+
 patternProperties:
   '^opp-table(-[a-z0-9]+)?$':
     if: