irqchip/apple-aic: Mark aic_init_smp() as __init

This function is only called from the driver init code.

Signed-off-by: Donghyeok Kim <dthex5d@gmail.com>
Acked-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211204164228.5920-1-dthex5d@gmail.com