irqchip/gic-v2m: Prevent use after free of gicv2m_get_fwnode()

With ACPI in place, gicv2m_get_fwnode() is registered with the pci
subsystem as pci_msi_get_fwnode_cb(), which may get invoked at runtime
during a PCI host bridge probe. But, the call back is wrongly marked as
__init, causing it to be freed, while being registered with the PCI
subsystem and could trigger:

 Unable to handle kernel paging request at virtual address ffff8000816c0400
  gicv2m_get_fwnode+0x0/0x58 (P)
  pci_set_bus_msi_domain+0x74/0x88
  pci_register_host_bridge+0x194/0x548

This is easily reproducible on a Juno board with ACPI boot.

Retain the function for later use.

Fixes: 0644b3da ("irqchip/gic-v2m: acpi: Introducing GICv2m ACPI support")
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org