software/ors-amarisoft: ru/lopcomm: Simplify and clarify cu_config.jinja2.xml

We can use mathematical relation in between ports, channels and tx/rx endpoints
to bring structure and clarity in RU setup.

Also, while on it, document how tx/rx data flow are organized as it was not
clear just from reading ORAN specs and I had to deduce it.

/cc @jhuge, @lu.xu, @xavier_thompson, @Daetalus
/reviewed-by @lu.xu
/reviewed-on nexedi/slapos!1468