PCI: exynos: Fix add_pcie_port() section mismatch warning

add_pcie_port() is called only from exynos_pcie_probe(), which is annotated
with __init.  Thus it makes sense to annotate add_pcie_port() with __init
to avoid the following section mismatch warning:

  WARNING: drivers/pci/built-in.o(.text.unlikely+0xf8): Section mismatch in reference from the function add_pcie_port() to the function .init.text:dw_pcie_host_init()
    The function add_pcie_port() references
    the function __init dw_pcie_host_init().
    This is often because add_pcie_port lacks a __init
    annotation or the annotation of dw_pcie_host_init is wrong.

[bhelgaas: changelog]
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>