regmap: irq: Allow using zero value for ack_base

In some cases, clear interrupt register may be at address 0.
This patch allows to use such configurations by adding additional
configuration bit to indicate this.

[With doc fix from Levente Kurusa <levex@linux.com> -- broonie]

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Mark Brown <broonie@linaro.org>