Add a new operation class for IPR accesses, and have IPR-accessing

Add a new operation class for IPR accesses, and have IPR-accessing
instructions use it (instead of IntALU, as before).  Default config
has a single non-pipelined 3-cycle unit.  A bit conservative for the
ev6 (some are 1, some are 3).

arch/alpha/isa_desc:
    Make hw_mfpr and hw_mtpr use IprAccessOp op class.
cpu/full_cpu/op_class.hh:
    Add IprAccess.

--HG--
extra : convert_revision : d4103da3343a586936839e29981fd15d6930d442