From: James Cleverdon <jamesclv@us.ibm.com>

Break out the definition of NR_IRQ_VECTORS, etc from irq_vectors.h into
irq_vectors_limits.h, so we can change it per subarch without having code
duplication for the rest of the file.  Stick the same values back for
mach-default, and override them for mach-summit/generic which needs bigger
limits.