PCI: designware-ep: Add multiple PFs support for DWC

Add multiple PFs support for DWC, due to different PF have different
config space, we use func_conf_select callback function to access
the different PF's config space, the different chip company need to
implement this callback function when use the DWC IP core and intend
to support multiple PFs feature.

Link: https://lore.kernel.org/r/20200918080024.13639-2-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>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>