powerpc/pseries: Track previous CPPR values to correctly EOI interrupts

At the moment when we EOI an interrupt we set the CPPR back to 0xFF
regardless of its previous value. This could lead to problems if we
take an interrupt with a priority of 5, but before EOIing it we get
an IPI which has a priority of 4. The problem is that at the moment
when we EOI the IPI we will set the CPPR to 0xFF, but it should
really be set back to 5 (the previous priority).

To keep track of the previous CPPR values we create the xics_cppr
structure that has an array for CPPR values and an index pointing
to the current priority. This can easily grow if new priorities get
added in the future.

This will also be useful because the partition adjunct option of
upcoming machines will update the H_XIRR hcall to accept the CPPR
as a parameter.

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>