PCI: mediatek: Add MSI support for MT2712 and MT7622

MT2712 and MT7622's PCIe host controller support MSI, but only 32-bit MSI
addresses are supported. It connects to GIC with the same IRQ number as the
INTx IRQ, so it shares the same IRQ with INTx IRQ.

Add MSI support for MT2712 and MT7622.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[bhelgaas: changes to follow rcar & tegra: rename to mtk_pcie_msi_alloc(),
add mtk_pcie_msi_free(), free hwirq if irq_create_mapping() fails, call
irq_dispose_mapping() from mtk_msi_teardown_irq()]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>