sh: NR_IRQS consolidation.

Each board sets the total number of IRQs that it's interested in via
the machvec. Previously we cared about the off vs on-chip IRQ range,
but any code relying on that is long dead. Set NR_IRQS to something
sensible given the vector range, and allow boards to cap it if they
really care.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>