loadstore: Convert to 3-stage pipeline

loadstore: Convert to 3-stage pipeline

This makes loadstore use a 3-stage pipeline.  For now, only one
instruction goes through the pipe at a time.  Completion and writeback
are still combinatorial off the valid signal back from the dcache, so
performance should be the same as before.  In future it should be able
to sustain one load or store per cycle provided they hit in the
dcache.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>