PCI: tegra: Add missing __iomem annotation

Added missing __iomem annotation in order to fix the following
sparse warnings:

drivers/pci/host/pci-tegra.c:411:41: warning: incorrect type in return expression (different address spaces)
drivers/pci/host/pci-tegra.c:411:41:    expected void [noderef] <asn:2>*
drivers/pci/host/pci-tegra.c:411:41:    got void *addr
drivers/pci/host/pci-tegra.c:419:25: warning: incorrect type in return expression (different address spaces)
drivers/pci/host/pci-tegra.c:419:25:    expected void [noderef] <asn:2>*
drivers/pci/host/pci-tegra.c:419:25:    got void *addr

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thierry Reding <treding@nvidia.com>