PCI: dra7xx: Iterate over INTx status bits

It is possible that more than one legacy IRQ may be set at the same
time, therefore iterate and handle all the pending INTx interrupts
before clearing the status and exiting the IRQ handler. Otherwise, some
interrupts would be lost.

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>