dt-bindings: clock: cs2000-cp: document aux-output-source

This new optional property can be used to control the function of the
auxiliary output pin. Introduce a new dt-bindings include file that
contains the numerical values.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220125093336.226787-3-daniel@zonque.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>