systemc: Remove some old warn includes from ext/channel.

The calls to sc_channel_warn_unimpl have been removed, so the includes
aren't needed.

Change-Id: I58a916a7c4ef9a266be591789696457799b6b8dd
Reviewed-on: https://gem5-review.googlesource.com/c/13198
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement the sc_signal_rv channel and ports.

Change-Id: Id1a3fd2ded224bbe94a4a65e0acf34a3547aedcc
Reviewed-on: https://gem5-review.googlesource.com/c/12813
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Stub out the predefined channels.

Change-Id: Ie030aad26875bd49e54981ec1e9076b7b5af6630
Reviewed-on: https://gem5-review.googlesource.com/10839
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>