dev: Convert the x86 i8237 DMA controller to use RegBank.

dev: Convert the x86 i8237 DMA controller to use RegBank.

This gets rid of the requirement to only modify one byte register at a
time, and builds some structure around individual DMA channels.

The one small feature of the i8237 that was implemented is still
implemented, but now with a method of the i8237.

Change-Id: Ibc2b2d75f2a3b860da3f28ae649c6f1a099bdf7d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36815
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>