sh: Kill off machvec IRQ hinting.

Everything is using sparseirq these days, so we have no need to
arbitrarily size nr_irqs ahead of time. The legacy IRQ pre-allocation
likewise has no meaning for us, so that's killed off too. We now depend
on nr_irqs expansion by the generic hardirq layer instead.

It's also worth noting that the majority of boards had completely bogus
values for their nr_irqs relative to their CPU and configurations, so
this ends up correcting behaviour for quite a few platforms.

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