pci: mediatek: add support for multiple ports in mediatek pcie gen3 driver

pci: mediatek: add support for multiple ports in mediatek pcie gen3 driver

One MediaTek PCIe Gen3 controller has only one port, where PCI bus 0
on this port represents the controller itself and bus 1 represents
the external PCIe device.

If multiple PCIe controllers are probed in U-Boot, U-Boot will use
bus numbers greater than 2 as input parameters. Therefore, we should
convert the BDF bus number to either 0 or 1 by subtracting the
offset by controller->seq_.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>