PCI: designware-ep: Modify MSI and MSIX CAP way of finding

PCI: designware-ep: Modify MSI and MSIX CAP way of finding

Each PF of EP device should have its own MSI or MSIX capabitily
struct, so create a dw_pcie_ep_func struct and move the msi_cap
and msix_cap to this struct from dw_pcie_ep, and manage the PFs
via a list.

Link: https://lore.kernel.org/r/20200918080024.13639-5-Zhiqiang.Hou@nxp.com
Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>