PCI: dra7xx: Use PCI_NUM_INTX

Use the PCI_NUM_INTX macro to indicate the number of PCI INTx interrupts
rather than the magic number 4. This makes it clearer where the number
comes from & what it relates to.

Based-on-similar-patches-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>