PCI: xgene-msi: Restructure handler setup/teardown

Another utterly pointless aspect of the xgene-msi driver is that
it is built around CPU hotplug. Which is quite amusing since this
is one of the few arm64 platforms that, by construction, cannot
do CPU hotplug in a supported way (no EL3, no PSCI, no luck).

Drop the CPU hotplug nonsense and just setup the IRQs and handlers
in a less overdesigned way, grouping things more logically in the
process.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20250708173404.1278635-13-maz@kernel.org