irqchip/armada-370-xp: Suppress unused-function warning

irqchip/armada-370-xp: Suppress unused-function warning

armada_370_xp_msi_reenable_percpu() is only defined when CONFIG_PCI_MSI is
enabled, and only called when SMP is enabled.

Without CONFIG_SMP, there are no callers, which results in a build time
warning instead:

drivers/irqchip/irq-armada-370-xp.c:319:13: error: 'armada_370_xp_msi_reenable_percpu' defined but not used [-Werror=unused-function]
  319 | static void armada_370_xp_msi_reenable_percpu(void) {}
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Mark the function as __maybe_unused to avoid adding more complexity
to the #ifdefs.

Fixes: 8ca61cde32c1 ("irqchip/armada-370-xp: Enable MSI affinity configuration")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240322125838.901649-1-arnd@kernel.org