Add VCHI driver to HEAD

[gonzo]

Summary

VCHI driver switched to BSD license more then a year ago and it's essential for GPU/audio support on RaspberryPi.

Test Plan

• Run VCHI userland demos
• Run quake3

[adrian]

So far:

• Please delete or #if 0 out the RCU stuff - you said on IRC that we're not using it. I'd just like to ensure that the minimum amount of stuff in the compat layer is used. Otherwise we may see some update start using code that was previously dead in list.h (and other compat code) that hasn't been tested.
• We need to go and ensure that all of the calls to the allocator functions handle NULLs - linux doesn't return NULL for small allocations and I've been bitten by drivers that don't check. (But - we can do this post-commit.)
• Does the kmalloc() glue require M_ZERO? It may be a performance boost if we're using it correctly everywhere. Actually, we're not using it in this patch - so #if 0 it out, remove M_ZERO from it.

Other than that, the general review is "grr linux code with no OS agnostic layer, sigh Linux." But it's a good starting point. I think we need to get the busdma stuff looked at in closer depth and I'll go hammer the heck out of it once it's in -HEAD.

[gonzo]

• I removed unused macroses/functions from list.h
• M_ZERO is required at the moment. mtx/cv initializers expect the memory to be zero-ed.

[adrian]

OK. Let's land this and then sort through the busdma things as people are using it.

[gonzo]

Committed in r278277