ARM64: PCI: ACPI support for legacy IRQs parsing and consolidation with DT code

To enable PCI legacy IRQs on platforms booting with ACPI, arch code should
include ACPI-specific callbacks that parse and set-up the device IRQ
number, equivalent to the DT boot path. Owing to the current ACPI core scan
handlers implementation, ACPI PCI legacy IRQs bindings cannot be parsed at
device add time, since that would trigger ACPI scan handlers ordering
issues depending on how the ACPI tables are defined.

To solve this problem and consolidate FW PCI legacy IRQs parsing in one
single pcibios callback (pending final removal), this patch moves DT PCI
IRQ parsing to the pcibios_alloc_irq() callback (called by PCI core code at
driver probe time) and adds ACPI PCI legacy IRQs parsing to the same
callback too, so that FW PCI legacy IRQs parsing is confined in one single
arch callback that can be easily removed when code parsing PCI legacy IRQs
is consolidated and moved to core PCI code.

Suggested-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Tomasz Nowicki <tn@semihalf.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>