ARM: dts: msm: add switch-source to msm8996 camera

Led flash is now triggered via a switch node (qcom,switch).
The switch-source is required in camera flash nodes in order to
fire flash.

Change-Id: I5af5705f893cc562a76d53bc769af95c6ac4fbba
Signed-off-by: Xu Han <hanxu@codeaurora.org>