ARM: dts: msm: Add r/w access for SFN in fsm9900

Wallclock driver should provide r/w access to a register for storage
and retrieval of the SFN (ie. Starting Frame Number) in all fsm9900
targets.

Change-Id: I75579cca002c9b3724e99cf43a5c3a652867e58a
Acked-by: Perry Randise <prandise@qti.qualcomm.com>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>