Nexus improvements

[Nathan Whitehorn]

I've been trying for the last few days to clean up some of our OF/FDT 
code, in particular related to reducing code duplication. There is a 
12-year-old comment in sparc64/sparc64/nexus.c that "this code should 
get into dev/ofw to some extent". Most of the code has been moved now 
and the PowerPC nexus device has been replaced by the pieces of the 
sparc64 one that now do live in dev/ofw. The following (almost entirely 
negative) patch moves sparc64 to use it as well. Since it is just a copy 
of the sparc64 code, this should involve no functional changes, and I 
will commit it in a few days unless someone raises an objection. The 
patch is only compile tested for the time being, so hardware tests would 
be appreciated.

Patch at: http://people.freebsd.org/~nwhitehorn/sparc64-nexus-unify.diff
-Nathan