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amdmsr(4) to make xf86-video-geode useable of NetBSD

The X.Org xf86-video-geode driver, written form AMD Geode CPUs with
graphics chips (e.g. some PC Engines ALIX boards), needs to access some
MSRs (model specific registers) on the AMD Geode CPU at runtime to set
the resolution etc.

On Linux this is done by opening a special file under /dev, then
lseeking to the "right position" and writing to the file.  On BSD, or
OpenBSD to be specific, a different approach had been choosen:  I wrote
a small driver, amdmsr(4) (which btw autoconfigures and attaches only on
AMD Geode CPUs) which allows access to the MSRs via ioctl(2) calls.
xf86-video-geode driver supports this upstream.

Since I (again) have a need for small devce running X, I want to add
amdmsr(4) to src/ and xf86-video-geode to xsrc/.

Doing it this way will also keep the xf86-video-geode driver
maintainable, since it's compatible with the other BSD that uses this

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