dt-bindings: soc: qcom: apr: Make qcom,protection-domain optional again

The protection domain functionality exists only in SoCs starting from
MSM8998 [1], while the APR bindings are also used on older platforms.

Commit 41288c30 ("ASoC: dt-bindings: qcom,apr: Split services to
shared schema") made the "qcom,protection-domain" required but it
should remain optional to avoid dtbs_check warnings on older platforms,
e.g.:

arch/arm64/boot/dts/qcom/apq8096-db820c.dtb:
  apr: service@3: 'qcom,protection-domain' is a required property
  From schema: Documentation/devicetree/bindings/soc/qcom/qcom,apr.yaml

[1]: https://lore.kernel.org/all/20200312120842.21991-1-sibis@codeaurora.org/

Fixes: 41288c30 ("ASoC: dt-bindings: qcom,apr: Split services to shared schema")
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221229151648.19839-2-stephan@gerhold.net

Documentation/devicetree/bindings/soc/qcom/qcom,apr-services.yaml

@@ -39,8 +39,8 @@ properties:
   qcom,protection-domain:
     $ref: /schemas/types.yaml#/definitions/string-array
     description: |
-      Protection domain service name and path for APR service
-      possible values are::
+      Protection domain service name and path for APR service (if supported).
+      Possible values are::
       "avs/audio", "msm/adsp/audio_pd".
       "kernel/elf_loader", "msm/modem/wlan_pd".
       "tms/servreg", "msm/adsp/audio_pd".
@@ -49,6 +49,5 @@ properties:
 
 required:
   - reg
-  - qcom,protection-domain
 
 additionalProperties: true