regmap: irq: allow to register one cell interrupt controllers

The change makes possible to use regmap-irq interface within drivers
of simple interrupt controllers, which don't have an option to handle
different interrupt types and thus have one cell interrupt controllers
described in device tree bindings.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>