rpmsg: qcom_smd: Access APCS through mailbox framework

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.

Reviewed-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
diff --git a/drivers/rpmsg/Kconfig b/drivers/rpmsg/Kconfig
index 65a9f6b..9b14090 100644
--- a/drivers/rpmsg/Kconfig
+++ b/drivers/rpmsg/Kconfig
@@ -39,6 +39,7 @@
 
 config RPMSG_QCOM_SMD
 	tristate "Qualcomm Shared Memory Driver (SMD)"
+	depends on MAILBOX
 	depends on QCOM_SMEM
 	select RPMSG
 	help