genirq: Add single IRQ reservation helper

For cases that wish to reserve a single IRQ at a given place simply
provide a wrapper in to the ranged reservation routine.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
LKML-Reference: <20101026071912.GD4733@linux-sh.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>