[rfc] mips74k/ar9344 support

[Adrian Chadd]

Hi!

Here's an initial hacked up patch to get the AR9344 to mountroot> .

(Which isn't true - it panics due to missing PHY setup on if_arge; I'll fix
that up soon.)

http://people.freebsd.org/~adrian/mips/20130929-mips74k-1.diff

The specific changes:

* add CPU_MIPS24KC and CPU_MIPS74KC
* mips32r2 CPUs require an EHB as a hazard for things, rather than NOPs
* mips74k cores have a different CCA (cache coherency attributes) than the
default - I lifted this from the netbsd mips support.

now, the ar71xx CPUs are all currently marked as CPU_MIPS4KC. I'll follow
this up with a commit to change that. There may be things we can optimise
in the CPU_MIPS24KC case.

Warner and I have started talking about how to properly fix all of the
hazard handling. We'll discuss that later.

I'd like to commit this to -HEAD so people wishing to hack on mips74k
platforms can do stuff. Does anyone have any issues with the above?

Thanks,



-adrian