PCI: iproc: Fix NULL pointer dereference for BCMA

With the inbound DMA mapping supported added, the iProc PCIe driver
parses DT property "dma-ranges" through call to
"of_pci_dma_range_parser_init()". In the case of BCMA, this results in a
NULL pointer deference due to a missing of_node.

Fix this by adding a guard in pcie-iproc-platform.c to only enable the
inbound DMA mapping logic when DT property "dma-ranges" is present.

Fixes: dd9d4e74 ("PCI: iproc: Add inbound DMA mapping support")
Reported-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Ray Jui <ray.jui@broadcom.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Tested-by: Rafał Miłecki <rafal@milecki.pl>
cc: <stable@vger.kernel.org> # 4.10+