tpm: transition tpm_vtpm_proxy documentation to the Sphinx

Transitioned the tpm_vtpm_proxy documentation to the Sphinx
infrastructure and removed parts from the documentation that are easier
to pull from the sources. Restructured vtpm_proxy.h and tpm_vtpm_proxy.c
to be compatible with this approach and wrote associated documentation
comments.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>