xivo-snom: add basic support for D765

Firmware (and documentation) for the D765 has not been released yet.
Once this get released, will be able to complete the support for the
D765, i.e.:

* removing the "update_policy" to "settings_only" for the D765
  (otherwise an error messages is displayed and the phone doesn't
  continue its booting sequence)
* adding the D765 to the description (plugin-info)
* adding the D765 in xivo documentation

The D765 on which I did some tests had its firmware in version 8.7.5.21.