ppc/spapr: Introduce SPAPR_IRQ_NR_IPIS to refer IRQ range for CPU IPIs.

ppc/spapr: Introduce SPAPR_IRQ_NR_IPIS to refer IRQ range for CPU IPIs.

spapr_irq_init currently uses existing macro SPAPR_XIRQ_BASE to refer to
the range of CPU IPIs during initialization of nr-irqs property.
It is more appropriate to have its own define which can be further
reused as appropriate for correct interpretation.

Suggested-by: Cedric Le Goater <clg@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Kowshik Jois <kowsjois@linux.ibm.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
(cherry picked from commit 2df5c1f5b014126595a26c6797089d284a3b211c)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>