sh: modify NR_IRQS from 256 to 512

Newer parts need NR_IRQS > 256, so simply bump this up to 512 across the
board. At this point sparseirq is used unconditionally across all CPUs,
so this introduces minimal overhead.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>