tpm, tpm_vtpm_proxy: add kdoc comments for VTPM_PROXY_IOC_NEW_DEV

Added kdoc comments for VTPM_PROXY_IOC_NEW_DEV so that these can be
imported to the kernel documentation written with rst markup and
generated with Sphinx.

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