tpm: Add documentation for the tpm_vtpm_proxy device driver

Add documentation for the tpm_vtpm device driver that implements
support for providing TPM functionality to Linux containers.

Parts of this documentation were recycled from the Xen vTPM
device driver documentation.

Update the documentation for the ioctl numbers.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

CC: linux-kernel@vger.kernel.org
CC: linux-doc@vger.kernel.org
CC: linux-api@vger.kernel.org
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>