PCI: mvebu: Implement support for legacy INTx interrupts

PCI: mvebu: Implement support for legacy INTx interrupts

This adds support for legacy INTx interrupts received from other PCIe
devices and which are reported by a new INTx irq chip.

With this change, kernel can distinguish between INTA, INTB, INTC and INTD
interrupts.

Note that for this support, device tree files has to be properly adjusted
to provide "interrupts" or "interrupts-extended" property with intx
interrupt source, "interrupt-names" property with "intx" string and also
'interrupt-controller' subnode must be defined.

If device tree files do not provide these nodes then driver would work as
before.

Link: https://lore.kernel.org/r/20220222155030.988-12-pali@kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>