regmap: irq: Allow to acknowledge masked interrupts during initialization

In case the hardware interrupt mask register does not prevent the chip level
irq from being asserted by the corresponding interrupt status bit, already
set interrupt bits should to be cleared once after masking them during
initialization. Add a flag to let drivers enable this behavior.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>