Bluetooth: ISO: Change CIG/BIG to use slist instead of array

A CIG may be updated later to include more CIS, and thus a slist
makes more sense.

The BIG doesn't need the change, but it makes more sense to
have similar handling for both.

This change also removes the requirement that the arrays
used to create the CIG/BIG need to static.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>