Add optional "moortec,vm-active-channels" property to define the number
of active channels per VM.

This shall be useful to avoid exposing sysfs for reading inputs that are
not connected to any voltage source.
Signed-off-by: Eliav Farber <farbere@amazon.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220908152449.35457-12-farbere@amazon.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>