x86/mm: Extend cc_attr to include AMD SEV-SNP

The CC_ATTR_GUEST_SEV_SNP can be used by the guest to query whether the
SNP (Secure Nested Paging) feature is active.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-10-brijesh.singh@amd.com