PCI: mediatek: Add loadable kernel module support

Implement remove() callback function for the Mediatek PCIe controller
driver to add loadable kernel module support.

Save the PCIe's GIC IRQ at probe so that it can be retrieved to
call dispose_irq() to tear down the IRQ upon module removal.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>