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

This reverts commit 421063ef.

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>