of/pci: Add support for parsing PCI host bridge resources from DT

Provide a function to parse the PCI DT ranges that can be used to create a
pci_host_bridge structure together with its associated bus.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
[make io_base parameter optional]
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Arnd Bergmann <arnd@arndb.de>
CC: Grant Likely <grant.likely@linaro.org>
CC: Rob Herring <robh+dt@kernel.org>
CC: Catalin Marinas <catalin.marinas@arm.com>