dev: Use regular atomic accesses for DMA in bypass mode.

dev: Use regular atomic accesses for DMA in bypass mode.

These are now accelerated with backdoor accesses and should be at least
as fast as functional accesses. This removes a dependency on port
proxies, and also stops the HDLCD from using functional accesses.

Change-Id: I5e959288eb533d09cffa7b79938aa2f61e4aff7d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38720
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>