PCI: Use u8 as in the spec, add port docs

[hecatia-elegua]

• also adds generic read/write methods for one of the ports

Please merge after #825.
I'm unsure if this is the right way to, like, do a follow-up PR?
I guess if the content is unrelated, I can start a completely different branch..
would be wild if one could specify that this PR depends on the previous one being done (which is very natural, similar to the git branch concept)

[kevinaboos]

Thanks, this looks pretty good! I'm not sure why we were using u16s instead of u8s in the first place.

One request -- I'm not sure how I feel about #825, I may not merge that in yet. But the changes from commit 815484f are generally good, and also independent from #825. So let's keep this PR strictly about u16 --> u8 such that we can address #825 separately.

Basically, please remove all changes related to #825, e.g., the determine_mem_base and BaseAddress thing.

[hecatia-elegua]

About #825: I guess you mean that the API for getting the base address is bad? I have more changes to come so it will not stay that way and I can read up on pci BARs to find out if returning an enum is sensible

[hecatia-elegua]

Sorry for the second force-push, now it should be good though. The docs are better too now.

[kevinaboos]

Thanks, everything looks good now.