irqchip/gic-v2m: Mark a few functions __init

They are all part of the init sequence.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221121140048.534395323@linutronix.de