core: Allow multiple loadstore instructions to be in flight

core: Allow multiple loadstore instructions to be in flight

The idea here is that we can have multiple instructions in progress at
the same time as long as they all go to the same unit, because that
unit will keep them in order.  If we get an instruction for a
different unit, we wait for all the previous instructions to finish
before executing it.  Since the loadstore unit is the only one that is
currently pipelined, this boils down to saying that loadstore
instructions can go ahead while l_in.in_progress = 1 but other
instructions have to wait until it is 0.

This gives a 2% increase on coremark performance on the Arty A7-100
(from ~190 to ~194).

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