dt-bindings: iio: Add ADC support for PM7325 and PM7325B

Add virtual channel definitions for PM7325 and PM7325B for
use by their ADC clients.

Change-Id: I75ddc15f6866a56678f344f6d18062857c4fc550
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>