dt-bindings: remoteproc: qcom,adsp: Allow cx-supply on qcom,sdm845-slpi-pas

One SDM845 board uses cx-supply, which is not allowed by the bindings,
as reported by dtbs_check:

sdm845-samsung-starqltechn.dtb: remoteproc@5c00000 (qcom,sdm845-slpi-pas): Unevaluated properties are not allowed ('cx-supply' was unexpected)

The SDM845 SLPI binding already allows lcx and lmx domains, thus the
cx-supply seems like a fake name for something else, e.g. some
enable pin. The qcom_q6v5_pas.c driver parses cx-supply, so it is an
established ABI, therefore document it for this device only.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Reviewed-by: Konrad Dybcio <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bjorn Andersson <[e-mail]>

Documentation/devicetree/bindings/remoteproc/qcom,adsp.yaml

···
   reg:
     maxItems: 1
 
+  cx-supply: true
+
   px-supply:
     description: Phandle to the PX regulator
 
···
           items:
             - const: lcx
             - const: lmx
+    else:
+      properties:
+        cx-supply: false
 
   - if:
       properties: