PCI: dra7xx: Fix legacy INTD IRQ handling

Legacy INTD IRQ handling is broken on dra7xx due to fact that driver
uses hwirq in range of 1-4 for INTA, INTD whereas IRQ domain is of size
4 which is numbered 0-3. Therefore when INTD IRQ line is used with
pci-dra7xx driver following warning is seen:

       WARNING: CPU: 0 PID: 1 at kernel/irq/irqdomain.c:342 irq_domain_associate+0x12c/0x1c4
       error: hwirq 0x4 is too large for dummy

Fix this by using pci_irqd_intx_xlate() helper to translate the INTx 1-4
range into the 0-3 as done in other PCIe drivers.

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Reported-by: Chris Welch <Chris.Welch@viavisolutions.com>
Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>