iio: xoadc: augment DT bindings a bit

In order to accommodate in a logical manner for the premuxed channels
in PM8921 and the similarly addressed channels in later PMICs, we
need a twocell arrangement with premux and analog mux setting as
a tuple to uniquely identify a hardware channel.

These bindings are not yet in use, so it should be fine to augment
them before we actually start using it in drivers and device trees.

This scheme came out of lengthy discussions and reverse-engineering
and reading of the few information sources we have.

Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh+dt@kernel.org>
Suggested-by: Björn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>