PCI: exynos: Refactor to make it easier to support other SoCs

Currently the Exynos PCIe driver only supports the Exynos5440 SoC.
Refactor the driver to allow support for other Exynos SoC.

Following are the main changes in this patch:

1) Add separate structs for memory, clock resources

  Future Exynos SoC will have different hardware resources such as iomem,
  clocks, regmap handles, etc., so keeping these resources in separate
  structs will let us initialize them via per-SoC ops and avoid littering
  the code with of_machine_is_compatible().

2) Add exynos_pcie_ops struct which will allow us to support the
   differences in resources in different Exynos SoC.

No functional change intended.

Signed-off-by: Niyas Ahmed S T <niyas.ahmed@samsung.com>
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Jingoo Han <jingoohan1@gmail.com>