PCI: generic: Add generic PCI host controller driver

Add support for a generic PCI host controller, such as a
firmware-initialised device with static windows or an emulation by
something such as kvmtool.

The controller itself has no configuration registers and has its address
spaces described entirely by the device-tree (using the bindings from
ePAPR).  Both CAM and ECAM are supported for Config Space accesses.

Add corresponding documentation for the DT binding.

[bhelgaas: currently uses the ARM-specific pci_common_init_dev() interface]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>