dt-bindings: iio: Add PMIC5 GEN3 ADC support

Add definitions for ADC channels for PMIC5 GEN3 ADC driver.
Add virtual channel definitions for PM5100, to be used
by ADC clients for PMIC5 GEN3.

Change-Id: I1180553c3ee697f994324c60ebfb4e996221108e
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>