pvclock (kvm_clock) support: where to attach

[Emile 'iMil' Heitor <imil%home.imil.net@localhost>]

I ported pvclock / kvm_clock from OpenBSD in order for Firecracker to
have an RTC. It's working but I'm not entirely sure where to attach it.

The device is x86 only so I added the source code in arch/x86/x86, and
for now I have it attached at cpufeaturebus which felt the more natural.

Thoughts?

Here's the code: 
https://github.com/NetBSD/src/commit/c09440be5aca7e16ce845c3ccbdfb47bac03fb63

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Emile `iMil' Heitor <imil@{home.imil.net,NetBSD.org}> | https://imil.net