PCI: mediatek: Add system PM support for MT2712 and MT7622

In order to reduce the PCIe power consumption in system suspend,
the PCI bus physical layer should be gated. On system resume, the PCIe
link should be re-established and the related control register values
should be restored.

Define suspend_noirq & resume_noirq callback functions to implement
PM system syspend hooks for the PCI host controller.

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>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>