soc: qcom: smp2p: Access APCS as mailbox client

Attempt to acquire the APCS IPC through the mailbox framework and fall
back to the old syscon based approach, to allow us to move away from
using the syscon.

CR-Fixed: 2330873
Change-Id: I31d8c62d4df3882769656b55e54d968acefde1d3
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>
Git-commit: e7180264
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


[deesin@codeaurora.org resolved trivial merge conflicts]
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>