PCI: mediatek: Add new method to get irq number

Use platform_get_irq_byname() to get the irq number
if the "interrupt-names" property is defined.

Link: https://lore.kernel.org/r/20210823032800.1660-4-chuanjia.liu@mediatek.com


Signed-off-by: Chuanjia Liu <chuanjia.liu@mediatek.com>
[lorenzo.pieralisi@arm.com: commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>