PCI: mediatek: Add controller support for MT2712 and MT7622

MT2712 and MT7622 using a new IP block of Gen2 controller which has two
root ports and shares the same probing flow with MT2701/MT7623.

Both MT2712 and MT7622 have the same per-port control registers, but
there are slight differences between them:

  - MT7622 has more clocks than MT2712.

  - MT7622 has shared control registers which are used to enable LTSSM and
    ASPM while MT2712 does not.

Add host controller support for MT2712/MT7622.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[bhelgaas: folded in fix from http://lkml.kernel.org/r/1502715868-17651-2-git-send-email-honghui.zhang@mediatek.com

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>