docs/specs/tpm: reST-ify TPM documentation

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-Id: <20200121152935.649898-7-stefanb@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>