Add code needed to get a functional PCI subsytem when using in
conjunction with upstream Linux guest (4.13+). Tested to work against
"e1000e" (network adapter, using MSI interrupts) as well as
"usb-ehci" (USB controller, using legacy PCI interrupts).

Based on "i.MX6 Applications Processor Reference Manual" (Document
Number: IMX6DQRM Rev. 4) as well as corresponding dirver in Linux
kernel (circa 4.13 - 4.16 found in drivers/pci/dwc/*)

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>