tpm: clear RAM when "memory overwrite" requested

tpm: clear RAM when "memory overwrite" requested

Note: the "Platform Reset Attack Mitigation" specification isn't
explicit about NVDIMM, since they could have different usages. It uses
the term "system memory" generally (and also "volatile memory RAM" in
its introduction). For initial support, I propose to consider
non-volatile memory as not being subject to the memory clear. There is
an on-going discussion in the TCG "pcclientwg" working group for
future revisions.

CPU cache clearing is done unconditionally in edk2 since commit
d20ae95a13e851 (edk2-stable201811).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>