drivers/tpm: add xen tpmfront interface

drivers/tpm: add xen tpmfront interface

This is a complete rewrite of the Xen TPM frontend driver, taking
advantage of a simplified frontend/backend interface and adding support
for cancellation and timeouts.  The backend for this driver is provided
by a vTPM stub domain using the interface in Xen 4.3.

Signed-off-by: Daniel De Graaf <[email protected]>
Acked-by: Matthew Fioravante <[email protected]>
Reviewed-by: Konrad Rzeszutek Wilk <[email protected]>
Acked-by: Peter Huewe <[email protected]>
Reviewed-by: Peter Huewe <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>