iommu/amd: Add missing function prototypes to fix -Wmissing-prototypes

Few exported functions from AMD IOMMU driver are missing prototypes.
They have declaration in arch/x86/events/amd/iommu.h but this file
cannot be included in the driver.  Add prototypes to fix W=1 warnings
like:

    drivers/iommu/amd/init.c:3066:19: warning:
        no previous prototype for 'get_amd_iommu' [-Wmissing-prototypes]
     3066 | struct amd_iommu *get_amd_iommu(unsigned int idx)

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200727183631.16744-1-krzk@kernel.org


Signed-off-by: Joerg Roedel <jroedel@suse.de>