PCI: dwc: Detect number of iATU windows

Currently the number of inbound and outbound iATU windows are determined
from DT properties. Unfortunately, there's 'num-viewport' for RC mode
and 'num-ib-windows' and 'num-ob-windows' for EP mode, yet the number of
windows is not mode dependent. Also, 'num-viewport' is not clear whether
that's inbound, outbound or both. We can probably assume it's outbound
windows as that's all RC mode uses.

However, using DT properties isn't really needed as the number of
regions can be detected at runtime by poking the iATU registers. The
basic algorithm is just writing a target address and reading back what
we wrote. In the unrolled ATU case, we have to take care not to go
past the mapped region.

With this, we can drop num_viewport in favor of num_ob_windows instead.

Link: https://lore.kernel.org/r/[email protected]
Tested-by: Marek Szyprowski <[email protected]>
Signed-off-by: Rob Herring <[email protected]>
Signed-off-by: Lorenzo Pieralisi <[email protected]>
Cc: Jingoo Han <[email protected]>
Cc: Gustavo Pimentel <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Jonathan Hunter <[email protected]>
Cc: [email protected]

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