PCI: imx6: Fix imx6_add_pcie_port() section mismatch warning

imx6_add_pcie_port() is called only from from imx6_pcie_probe() which is
annotated with __init.  Thus it makes sense to annotate
imx6_add_pcie_port() with __init to avoid section mismatch warnings.

[bhelgaas: changelog]
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Sean Cross <xobs@kosagi.com>