systemc: Make sc_out delegate to its base class sc_inout.

systemc: Make sc_out delegate to its base class sc_inout.

The spec says these are essentially identical classes, they just have
both so that users can show their intent.

Change-Id: I51908edca89acea25891a52bfa7fca0681ccfc5c
Reviewed-on: https://gem5-review.googlesource.com/12220
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>