mips: Carefully use device_set_private for cpuN.

But don't do it in cpu_attach_common because the callers aren't set
up right -- instead leave a comment about what's wrong, to be dealt
with later.

Provide and use some CP0 accessor functions instead of M[TF]C0 macros
for readability.

While here convert some other M[TF]C0 uses to already exising accessor
functions, e.g. mipsNN_cp0_ebase_read

Trailing whitespace

#include "opt_multiprocessor.h"

first shot at SMP support, very much broken and experimental
So far the 2nd core wakes up, makes its way to the idle loop, and things lock
up when we start the timer interrupt.