PPC: Set MPIC IDE for IPI to 0

We use the IDE register with IPIs as a mask to keep track which processors
have already acknowledged the respective interrupt. So we need to initialize
it to 0 to make sure that it doesn't accidently fire an IPI on CPU0 when the
first IPI is triggered.

Reported-by: Elie Richa <richa@adacore.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

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v2 -> v3:

  - fix IDE IPI reset
