systemc: Add Port types for initiator and target sockets.

These make it clear that a port represents a TLM socket, what direction
it faces, and what its width is.

Change-Id: Idcbea2b9b053f1e0685e011bc2c7de2468fb25b2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18172
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>