PCI: mediatek-gen3: Add mtk_gen3_pcie_pdata data structure

PCI: mediatek-gen3: Add mtk_gen3_pcie_pdata data structure

Introduce mtk_gen3_pcie_pdata data structure in order to define
multiple callbacks for each supported SoC.

This is a preliminary patch to introduce EN7581 PCIe support.

Link: https://lore.kernel.org/linux-pci/c193d1a87505d045e2e0ef33317bce17012ee095.1720022580.git.lorenzo@kernel.org
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Tested-by: Zhengping Zhang <zhengping.zhang@airoha.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Jianjun Wang <jianjun.wang@mediatek.com>