irqchip/armada-370-xp: Refactor handling IPI interrupts

irqchip/armada-370-xp: Refactor handling IPI interrupts

Refactor the handling of IPI interrupts
- put into own function mpic_handle_ipi_irq(), similar to
  mpic_handle_msi_irq()
- rename the variable holding the doorbell cause register to "cause"
- retype and rename the variable holding the IPI HW IRQ number to
  "irq_hw_number_t i"

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/all/20240711115748.30268-11-kabel@kernel.org