dt-bindings: mailbox: qcom: add SDX55 compatible

The commit 0d17014e ("dt-bindings: mailbox: Add binding for SDX55
APCS") added SDX55 compatible string to one of clock-selection
conditions, but failed to add one to the main schema's compatible list.
Fix this omission.

Fixes: 0d17014e ("dt-bindings: mailbox: Add binding for SDX55 APCS")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/qcom,apcs-kpss-global.yaml

@@ -39,6 +39,7 @@ properties:
               - qcom,msm8953-apcs-kpss-global
               - qcom,msm8994-apcs-kpss-global
               - qcom,qcs404-apcs-apps-global
+              - qcom,sdx55-apcs-gcc
           - const: syscon
   reg:
     maxItems: 1