iommu/amd: Introduce function to check and enable SNP

To support SNP, IOMMU needs to be enabled, and prohibits IOMMU
configurations where DTE[Mode]=0, which means it cannot be supported with
IOMMU passthrough domain (a.k.a IOMMU_DOMAIN_IDENTITY),
and when AMD IOMMU driver is configured to not use the IOMMU host (v1) page
table. Otherwise, RMP table initialization could cause the system to crash.

The request to enable SNP support in IOMMU must be done before PCI
initialization state of the IOMMU driver because enabling SNP affects
how IOMMU driver sets up IOMMU data structures (i.e. DTE).

Unlike other IOMMU features, SNP feature does not have an enable bit in
the IOMMU control register. Instead, the IOMMU driver introduces
an amd_iommu_snp_en variable to track enabling state of SNP.

Introduce amd_iommu_snp_enable() for other drivers to request enabling
the SNP support in IOMMU, which checks all prerequisites and determines
if the feature can be safely enabled.

Please see the IOMMU spec section 2.12 for further details.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Co-developed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Link: https://lore.kernel.org/r/20220713225651.20758-7-suravee.suthikulpanit@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>