tpm: move TpmIf in include/sysemu/tpm.h

This is a better location than hw/tpm, since we are going to use the
interface from outside hw/tpm.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>