PCI: exynos: Add I/O access wrappers

This patch adds wrappers for MMIO access to ELBI, PHY, and other
registers.  No functional change.

[bhelgaas: changelog]
Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>