x86/cpufeatures: Add SNP Secure TSC

x86/cpufeatures: Add SNP Secure TSC

The Secure TSC feature for SEV-SNP allows guests to securely use the RDTSC
and RDTSCP instructions, ensuring that the parameters used cannot be
altered by the hypervisor once the guest is launched. For more details,
refer to the AMD64 APM Vol 2, Section "Secure TSC".

Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Tested-by: Vaishali Thakkar <vaishali.thakkar@suse.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Link: https://lore.kernel.org/r/20250819234833.3080255-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>