[MIPS] Added missing cases for rdhwr emulation

Some of these are architecturally required for R2 processors so lets try
to be bit closer to the real thing.  This also provides access to the
CPU cycle timer, even on multiprocessors.  In that aspect its currently
bug compatible to what would happen on a R2-based SMP.

Signed-off-by: Chris Dearman <chris@mips.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>