powerpc: Increase NR_IRQS range to support more KVM guests

PowerNV systems can handle up to 4K guests and 1M interrupt numbers
per chip. Increase the range of allowed interrupts to support a larger
number of guests.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201210171450.1933725-8-clg@kaod.org