pnv/xive: Add special handling for pool targets

pnv/xive: Add special handling for pool targets

Hypervisor "pool" targets do not get their own interrupt line and instead
must share an interrupt line with the hypervisor "physical" targets.
This also means that the pool ring must use some of the registers from the
physical ring in the TIMA.  Specifically, the NSR, PIPR and CPPR registers:

  NSR = Notification Source Register
  PIPR = Post Interrupt Priority Register
  CPPR = Current Processor Priority Register

The NSR specifies that there is an active interrupt.  The CPPR
specifies the priority of the context and the PIPR specifies the
priority of the interrupt.  For an interrupt to be presented to
a context, the priority of the interrupt must be higher than the
priority of the context it is interrupting (value must be lower).

The existing code was not aware of the sharing of these registers.
This commit adds that support.

Signed-off-by: Glenn Miles <milesg@linux.vnet.ibm.com>
Signed-off-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>