uninorth: use qdev gpios for PCI IRQs

Currently an object link property is used to pass a reference to the OpenPIC
into the PCI host bridge so that pci_unin_init_irqs() can connect the PCI
IRQs to the PIC itself.

This can be simplified by defining the PCI IRQs as qdev gpios and then wiring
up the PCI IRQs to the PIC in the New World machine init function.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201013114922.2946-4-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>