ARM: dts: msm: Move MSM8994 VADC channels to board CDP file

The MSM8994 Interposer CDP requires the presence of the VADC channels,
they are currently defined in MSM8994 thus making them unreachable.

Move the channel definitions to the corresponding MSM8994 CDP device
tree file to make them available for the MSM8994 interposer CDP.

Change-Id: I8a2b4a8a9db3eba92868e7781ad4c4ba06d7cc65
Signed-off-by: David Keitel <dkeitel@codeaurora.org>