iommu/amd: Add amd_iommu=force_enable option

Add this option to enable the IOMMU on platforms like AMD Stoney,
where the kernel usually disables it because it may cause problems in
some scenarios.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20210603130203.29016-1-joro@8bytes.org