allocate_irqno, free_irqno and alloc_legacy_irqno are a simple allocator
for interrupt numbers from the days when the numer of interrupts was still
fixed to NR_IRQS.  This was necessary for the SGI IP27 architecture which
with its flexible architecture and possibly large number of interrupts
doesn't easily fit into the old pattern.  These days there are better
alternatives.

Move the allocation code from the arch generic code to the only platform
using it, the SGI IP27 aka Origin 200/2000, Onyx 2.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>