systemc: Teach the TLM bridges how to use gem5's new backdoor mechanism.

systemc: Teach the TLM bridges how to use gem5's new backdoor mechanism.

This change teaches the TLM bridges to translate between TLM's DMI
mechanism and gem5's backdoor mechanism.

Change-Id: I942a6cce4fb87f10e8173f4ee49b6c7b0ffa7e4a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17591
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>