arm/dt: camera: Add LED camera flash node in dtsi

Add LED camera flash device node for 8974 that uses pm8941 LED flash
source. Add document for msm-camera-flash describing the required
properties and its significance.

Change-Id: Ic0ecfc547fead58df6fc19839dc1a7c5587e9b1f
Signed-off-by: Sreesudhan Ramakrish Ramkumar <srramku@codeaurora.org>