sh: intc: Kill off special reservation interface.

At present reserving the IRLs in the IRQ bitmap in addition to the
dropping of the legacy IRQ pre-allocation prevent IRL IRQs from being
allocated for the x3proto board.

The only reason to permit reservations was to lock down possible hardware
vectors prior to dynamic IRQ scanning, but this doesn't matter much given
that the hardware controller configuration is sorted before we get around
to doing any dynamic IRQ allocation anyways. Beyond that, all of the
tables are __init annotated, so quite a bit more work would need to be
done to support reconfiguring things like IRL controllers on the fly,
much more than would ever make it worth the hassle.

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