PCI: mediatek-gen3: Add MediaTek Gen3 driver for MT8192

MediaTek's PCIe host controller has three generation HWs, the new
generation HW is an individual bridge, it supports Gen3 speed and
compatible with Gen2, Gen1 speed.

Add support for new Gen3 controller which can be found on MT8192.

Link: https://lore.kernel.org/r/20210420061723.989-4-jianjun.wang@mediatek.com


Signed-off-by: Jianjun Wang <jianjun.wang@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>