PCI: dwc: dra7xx: Invoke phy_set_mode() API to set PHY mode to PHY_MODE_PCIE

Certain PHYs used with PCIe controller can also be used with other
controllers such as USB or SATA. In order to configure the PHY
to work with PCIe controller, invoke phy_set_mode() API with mode
set to PHY_MODE_PCIE.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>