dt-bindings: mailbox: qcom: enable syscon compatible for msm8976

On msm8976 platform APCS also uses syscon compatible, so move it to the
block of compatibles using SoC-compat together with syscon.

Fixes: 60545466 ("dt-bindings: mailbox: qcom,apcs-kpss-global: Add syscon const for relevant entries")
Acked-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>