PCI: dra7xx: Rename add_pcie_port() to dra7xx_add_pcie_port()

The add_pcie_port() function is dra7xx-specific.  Add dra7xx prefix to
avoid collision in global name space.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>