PCI: mediatek-gen3: Add system PM support

PCI: mediatek-gen3: Add system PM support

Add suspend_noirq and resume_noirq callback functions to implement PM
system suspend and resume hooks for the MediaTek Gen3 PCIe controller.

When the system suspends, trigger the PCIe link to enter the L2 state
and pull down the PERST# pin, gating the clocks of the MAC layer, and
then power-off the physical layer to provide power-saving.

When the system resumes, the PCIe link should be re-established and the
related control register values should be restored.

Link: https://lore.kernel.org/r/20210420061723.989-7-jianjun.wang@mediatek.com
Signed-off-by: Jianjun Wang <jianjun.wang@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>