Build TPM passthrough for i386 and x86_64 targets

Build the TPM passthrough driver only for i386 and x86_64 targets
using the default-configs files for those targets with softmmu.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Reviewed-by: Joel Schopp <jschopp@linux.vnet.ibm.com>
Message-id: 1361987275-26289-8-git-send-email-stefanb@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>