ARM: dts: msm: Add PWM support for MPP LED

MPP connected WLED on MSM8x10 supports PWM to
provide brightness change. Add pwm parameters
such as channel and period to MPP 3 device node.

CRs-Fixed: 496357
Change-Id: I81c8758858e31a7db00e45f42db50d5bd67766c6
Signed-off-by: Chun Zhang <chunz@codeaurora.org>
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>