PCI: mediatek: Add a structure to abstract the controller generations

Introduce a structure "mtk_pcie_soc" to abstract the differences between
controller generations, and the .startup() hook is used to encapsulate some
SoC-dependent related setting.  In doing so, the common code which will be
reused by future chips.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>