systemc: Expose the utility functions for payload->packet conversion.

These can be used anywhere someone needs to turn a packet into a
payload or vice-versa.

Change-Id: I26a1f134e6fbec81b230a2853cf79264e80bf2dc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20859
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
3 files changed