Create a new attribute group meant to provide a single place that
defines endpoint IDs that might be needed by user space.  Not all
defined endpoints are presented, and only those that are defined
will be made visible.

The new attributes use "extended" device attributes to hold endpoint
IDs, which is a little more compact and efficient.  Reimplement the
existing modem endpoint ID attribute files using common code.
Signed-off-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20220719191639.373249-1-elder@linaro.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>