PCI: dwc: Move config space capability search API

Move PCIe config space capability search API to common DesignWare file
as this can be used by both host and EP mode drivers.

Signed-off-by: Vidya Sagar <vidyaos@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>