Add support for inbound DMA mapping.  The range of the inbound mapping is
configured by the optional device tree property 'dma-ranges'.

While inbound mapping is done automatically in the ASIC on most iProc-based
SoCs, newer ASICs (e.g., Stingray) require inbound mapping to be configured
explicitly in software.

[bhelgaas: fold in fixes to avoid 32-bit division in iproc_pcie_ib_write()
and uninitialized return value in iproc_pcie_setup_ib() from Arnd Bergmann
<arnd@arndb.de>]
Signed-off-by: Oza Oza <oza.oza@broadcom.com>
Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>