x86/sev: Share the sev_secrets_pa value again

x86/sev: Share the sev_secrets_pa value again

This commits breaks SNP guests:

  234cf67fc3bd ("x86/sev: Split off startup code from core code")

The SNP guest boots, but no longer has access to the VMPCK keys needed
to communicate with the ASP, which is used, for example, to obtain an
attestation report.

The secrets_pa value is defined as static in both startup.c and
core.c. It is set by a function in startup.c and so when used in
core.c its value will be 0.

Share it again and add the sev_ prefix to put it into the global
SEV symbols namespace.

[ mingo: Renamed to sev_secrets_pa ]

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Dionna Amalie Glaze <dionnaglaze@google.com>
Cc: Kevin Loughlin <kevinloughlin@google.com>
Link: https://lore.kernel.org/r/cf878810-81ed-3017-52c6-ce6aa41b5f01@amd.com