PCI: dwc: Add generic MSG TLP support for sending PME_Turn_Off when system suspend

Instead of relying on the vendor specific implementations to send the
PME_Turn_Off message, introduce a generic way of sending the message using
the MSG TLP.

This is achieved by reserving a region for MSG TLP of size
'pci->region_align', at the end of the first IORESOURCE_MEM window of the
host bridge. And then sending the PME_Turn_Off message during system
suspend with the help of iATU.

The reason for reserving the MSG TLP region at the end of the
IORESOURCE_MEM is to avoid generating holes in between, because when the
region is allocated using allocate_resource(), memory will be allocated
from the start of the window. Later, if memory gets allocated for an
endpoint of size bigger than 'region_align', there will be a hole between
MSG TLP region and endpoint memory.

This generic implementation is optional for the glue drivers and can be
overridden by a custom 'pme_turn_off' callback.

Link: https://lore.kernel.org/linux-pci/[email protected]
Signed-off-by: Frank Li <[email protected]>
Signed-off-by: Krzysztof Wilczyński <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Manivannan Sadhasivam <[email protected]>

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