PCI: exynos: Add exynos prefix to add_pcie_port()/pcie_init()

The add_pcie_port() and pcie_init() functions are Exynos-specific.  Add
exynos 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>