sh: Switch dynamic IRQ creation to generic irq allocator.

Now that the genirq code provides an IRQ bitmap of its own and the
necessary API to manipulate it, there's no need to keep our own version
around anymore.

In the process we kill off some unused IRQ reservation code, with future
users now having to tie in to the genirq API as normal.

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