PPC: Fix IPI support in MPIC

PPC: Fix IPI support in MPIC

The current IPI support in the MPIC code is incomplete and doesn't work. This
code adds proper support for IPIs in MPIC by using the IDE register to remember
which CPUs IPIs are still outstanding to. New triggers through the IPI trigger
register only add to the list of CPUs we want to IPI.

Signed-off-by: Alexander Graf <agraf@suse.de>
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v1 -> v2:

  - Use MAX_IPI instead of hardcoded 4

Signed-off-by: Alexander Graf <agraf@suse.de>