x86/sev: Add the Secure TSC feature for SNP guests

Now that all the required plumbing is done for enabling Secure TSC, add it to
the SNP features present list.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Tested-by: Peter Gonda <pgonda@google.com>
Link: https://lore.kernel.org/r/20250106124633.1418972-14-nikunj@amd.com