iommu/amd: Set AMD iommu callbacks for platform bus driver

AMD has more drivers will use ACPI to platform bus driver later,
all those devices need iommu support, for example: eMMC driver.

For latest AMD eMMC controller, it will utilize sdhci-acpi.c driver,
which will rely on platform bus to match device and driver, where we
will set 'dev' of struct platform_device as map_sg parameter passing
to iommu driver for DMA request, so the iommu-ops are needed on the
platform bus.

Signed-off-by: Wan Zongshun <Vincent.Wan@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>