Review request -- splitting OF enumeration from nexus

[Nathan Whitehorn]

Nexus on OF platforms doesn't behave like nexus on x86, which generates 
some periodic difficulty with cryptosoft or syscons attaching to all 
devices and taking over the system when someone makes a wrong 
assumption. I have done some work to split out OF enumeration into a 
new, acpi(4)-like bus called ofwbus that does all of the OF enumeration 
previously done by nexus(4). The patch can be found at 
http://people.freebsd.org/~nwhitehorn/ofwbus.diff.

Doing this also provides a number of other benefits: it shares code 
between PowerPC and sparc64, unifies the AIM and Book-E nexus 
implementations on PPC, and makes it easier to have non-Open Firmware 
platforms on PPC (the original motivation for the work). I have tested 
this code with no obvious problems on a variety of Apple PPC machines 
and a Sun Ultra 5. More testing and comments would be much appreciated. 
If no has any objections, I will commit these changes in 2 weeks.
-Nathan