systemc: Send response to TLM side if a packet does not need response

systemc: Send response to TLM side if a packet does not need response

A completed TLM transaction includes request and response parts.
Currently, if a gem5 packet does not need a reponse, the bridge would not
send BEGIN_RESP to its upstream. It causes stuck on TLM side.

To fix this problem, the bridge should send BEGIN_RESP by itself in this
case.

Change-Id: I318dec21bc3f291693715c0d70bc624addf05076
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32735
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabeblack@google.com>