axi_pkg: Add fix for vivado not correctly extending enum with all 1, note in doc connecting multiple axi_xbar

Use the macros in `typedef.svh` as replacement.

Modport `in` was an alias of `Slave` and `out` was an alias of `Master`.
However, many tools do not recognize such aliases as being identical,
causing problems when feeding a `Master` to a `out` (and vice-versa).  To
prevent these problems, the redundant `in` and `out` modports have been
removed.

This fixes issue #8.