tpm: Implement tpm_sized_buffer_reset

Move the definition of TPMSizedBuffer out of tpm_tis.c into tpm_util.h
and implement tpm_sized_buffer_reset() for the following patches to use.

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