PCI: layerscape: Add suspend/resume for ls1043a

Add suspend/resume support for Layerscape LS1043a.

In the suspend path, PME_Turn_Off message is sent to the endpoint to
transition the link to L2/L3_Ready state. In this SoC, there is no way to
check if the controller has received the PME_To_Ack from the endpoint or
not. So to be on the safer side, the driver just waits for
PCIE_PME_TO_L2_TIMEOUT_US before asserting the SoC specific PMXMTTURNOFF
bit to complete the PME_Turn_Off handshake. Then the link would enter L2/L3
state depending on the VAUX supply.

In the resume path, the link is brought back from L2 to L0 by doing a
software reset.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Frank Li <[email protected]>
Signed-off-by: Lorenzo Pieralisi <[email protected]>
Reviewed-by: Manivannan Sadhasivam <[email protected]>
Acked-by: Roy Zang <[email protected]>

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