PCI: of: Reduce missing non-prefetchable memory region to a warning

The pci-rcar-gen2 controller requires only a prefetchable memory region,
and the error prevents using pci_parse_request_of_pci_ranges() for it.
Let's reduce this to just a warning message so this function can be used
for pci-rcar-gen2.

Link: https://lore.kernel.org/r/20200722022514.1283916-17-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>