platform/x86: think-lmi: Add certificate as mechanism

As both password or certificate authentication are available as mechanisms
update the documentation to add certificate as an option

Update driver to return correct mechanism appropriately.

Signed-off-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Link: https://lore.kernel.org/r/20241024195536.6992-2-mpearson-lenovo@squebb.ca


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>