msm: pp2s: Add register access for SFN

Add new register access to store/retrieve SFN (ie. Starting Frame
Number).  This access is through sysfs.

Change-Id: Ie7e1ba4ec7778eeeee3e8a5f66108ec962053be0
Acked-by: Perry Randise <prandise@qti.qualcomm.com>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>