Revert "pci: msm: msm_pcie_iatu_config API clean"

This reverts commit b9b4ade9.
Older ver of SNPS core needs viewport registers as it does not
have separate iATU region. Hence keep the viewport support.

Change-Id: Ic863eb3332c54df433726064591a01dc50d29565
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>