regmap-irq: Remove support for not_fixed_stride

No remaining users, use a custom .get_irq_reg() callback instead.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Link: https://lore.kernel.org/r/20230511091342.26604-3-aidanmacdonald.0x0@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>