regmap: irq: add chip option mask_writeonly

Some irq controllers have writeonly/multipurpose register layouts. In
those cases we read invalid data back. Here we add the option
mask_writeonly as masking option.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>