ARM: dts: msm: Add VADC channels

Add USB DP/DM channel configuration to read
the D+/D- voltage level from the PMI8994 VADC.

Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Change-Id: I48ea05a9b9647fafb35e1c29293e372f82f0602f