pci: Add generic PCIe host bridge

With simple exposure of MMFG, ioport window, mmio window and an IRQ line we
can successfully create a workable PCIe host bridge that can be mapped anywhere
and only needs to get described to the OS using whatever means it likes.

This patch implements such a "generic" host bridge. It handles 4 legacy IRQ
lines. MSIs need to be handled external to the host bridge.

This device is particularly useful for the "pci-host-ecam-generic" driver in
Linux.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Tested-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
