PCI: mediatek: Add MediaTek PCIe host controller support

Add support for the MediaTek PCIe Gen2 controller which can be found on
MT7623 series SoCs.

[bhelgaas: fold in mtk_pcie_parse_and_add_res() bugfix from
http://lkml.kernel.org/r/1496644078-27122-1-git-send-email-ryder.lee@mediatek.com]
[bhelgaas: fold in MAINTAINERS update from
http://lkml.kernel.org/r/1497588789-28607-1-git-send-email-ryder.lee@mediatek.com]
[bhelgaas: fold in pci_scan_root_bus_bridge() update and leak fix from
http://lkml.kernel.org/r/1498555451-55073-2-git-send-email-ryder.lee@mediatek.com]
[bhelgaas: fold in powerup fixes from
http://lkml.kernel.org/r/1497866400-41844-2-git-send-email-ryder.lee@mediatek.com]
[bhelgaas: fold in poweroff when link down fixes from
http://lkml.kernel.org/r/1497866400-41844-3-git-send-email-ryder.lee@mediatek.com]
[bhelgaas: fold in optional property fixes from
http://lkml.kernel.org/r/1497866400-41844-4-git-send-email-ryder.lee@mediatek.com]
[bhelgaas: set host->map_irq and host->swizzle_irq and drop
pci_fixup_irqs(), remove unnecessary "return", rename mtk_pcie_link_is_up()
to mtk_pcie_link_up() for consistency, add local struct device pointer]
[bhelgaas: fold in pci_add_flags() removal from
http://lkml.kernel.org/r/1499061300-55951-1-git-send-email-ryder.lee@mediatek.com]
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>