dev: Make DMA devices use their own ports for functional accesses.

dev: Make DMA devices use their own ports for functional accesses.

DMA devices already have ports they use for non-functional accesses. We
can just attach a port proxy to that instead of getting one from the
system object.

Change-Id: I5e9adee43c7fe07b4c90978dbb7ec71468caadbb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38481
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexandru Duțu <alexandru.dutu@amd.com>