systemc: Make TLM port wrappers for tlm_base_(target|initiator)_socket.

These are useful if using TLM sockets without using the standard TLM
protocol. For instance, when used with ARM's fast models, this can wrap
sockets which carry the opaque GICv3Comms protocol.

Change-Id: I329a919068f958abbde2cb83683d3a3ae2e05a20
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20860
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

mem, sim, systemc: Reorganize Port and co.s bind, unbind slightly.

The base Port class can keep track of its peer, and also whether it's
connected. This is partially delegated away from the port subclasses
which still keep track of a cast version of their peer pointer for
their own conveneince, so that it can be used by generic code. Even
with the Port mechanism's new flexibility, each port still has
exactly one peer and is either connected or not based on whether there
is a peer currently.

Change-Id: Id3228617dd1604d196814254a1aadeac5ade7cde
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20232
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Provide a utility Port TLM socket wrapper class.

This class should make it easy to attach TLM sockets into the gem5
python Port config mechanism.

Change-Id: I3548c654d56bfc852f3fc98fe6433163b14f7fe3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17230
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>