regmap-irq: Add get_irq_reg() callback

Replace the internal sub_irq_reg() function with a public callback
that drivers can use when they have more complex register layouts.
The default implementation is regmap_irq_get_irq_reg_linear(), used
if the chip doesn't provide its own callback.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Link: https://lore.kernel.org/r/20220623211420.918875-12-aidanmacdonald.0x0@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>