drivers: soc: Add sdx ipc driver to communicate with external AP

Add sdx ipc driver to facilitate communication between external AP
and modem using IPC gpios. Modem is intimated of AP's state based
on gpio events and vice-versa.

Change-Id: Ic9c0a0688a1e136d8da5bbc5e6554194b3b2af01
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>
Signed-off-by: Manohar Vavilapalli <mvavilap@codeaurora.org>