mfd: qcom-pm8008: Use .get_irq_reg() for irq chip

Replace the deprecated not_fixed_stride flag and the associated
hierarchy of offsets with a .get_irq_reg() callback.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230216222214.138671-4-aidanmacdonald.0x0@gmail.com