systemc: Explicitly keep the sc_port bind alongside sc_in's version.

systemc: Explicitly keep the sc_port bind alongside sc_in's version.

The sc_in bind function was hiding the one from sc_port by changing
the const-ness of its parameter. This change explicitly exposes the
underlying sc_port version, and keeps it alongside the new sc_in
version.

This seems mildly dangerous and undesirable because now there are two
very similar functions which would both need to be overridden in order
to get new behavior, but I don't think it's any more dangerous and
undesirable than as (perhaps unintentionally) specified in the
standard.

Change-Id: Ib42a1f8e70bc97abeeeb8d614e71c4019b3a2323
Reviewed-on: https://gem5-review.googlesource.com/c/13880
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>