First cut at hwpmc support on MIPS

[George Neville-Neil]

Hi,

I've got this running on my RouterStation Pro, which is a MIPS 24K (32bit) CPU.
Please review and let me know if you have comments.  I'd like to commit this
in a week or so.

Best,
George

http://people.freebsd.org/~gnn/mipshwpmc_1.diff