ArmVirtPkg: implement FdtPciHostBridgeLib

ArmVirtPkg: implement FdtPciHostBridgeLib

Implement PciHostBridgeLib for DT platforms that expose a PCI root bridge
via a pci-host-ecam-generic DT node. The DT parsing logic is copied from
the PciHostBridgeDxe implementation in ArmVirtPkg, with the one notable
difference that we don't set some of the legacy PCI attributes for IDE
and VGA I/O ranges.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Ref: https://tianocore.acgmultimedia.com/show_bug.cgi?id=65