spmi: msm: Add virtual SPMI PMIC front end driver

Add basic infrastructure for SPMI front end driver. Frontend driver
communicates with backend to service SPMI request from clients.

Change-Id: Icff567a2f224fb49f370bf760287d9d3615a3325
Signed-off-by: Yimin Peng <yiminp@codeaurora.org>