regmap-irq: Deprecate the not_fixed_stride flag

This flag is a bit of a hack and the same thing can be accomplished
using a custom ->get_irq_reg() callback. Add a warning to catch any
use of the flag.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Link: https://lore.kernel.org/r/20220623211420.918875-13-aidanmacdonald.0x0@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>