Revert "PCI: dwc/keystone: Drop duplicated 'num-viewport'"

Revert "PCI: dwc/keystone: Drop duplicated 'num-viewport'"

This reverts commit e441e5b2ba1f97be9bae30ebd2f13386479b360c.

In preparation to detect the number of iATU regions instead of using DT
properties, we need to keep reading 'num-viewport' for the Keystone
driver which doesn't use the iATU in older versions of the IP.

However, note that Keystone has been broken for some time with upstream
dts files which don't set 'num-viewports'. The reverted commit did
make the property optional, but now it's mandatory again.

Link: https://lore.kernel.org/r/20201105211159.1814485-15-robh@kernel.org
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>