dt-bindings: mailbox: qcom: Add SM6115 APCS compatible

Add compatible for the Qualcomm SM4250/6115 APCS block to the Qualcomm
APCS binding.

Signed-off-by: Iskren Chernev <iskren.chernev@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>