mailbox: qcom: Add support for SM6115 APCS IPC

Qcom SM4250/6115, have APCS mailbox setup similar to msm8998 and
msm8916.

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