Documentation: tpm_tis

Based recent discussions on LKML, provide preliminary bits of tpm_tis_core
dependent drivers. Includes only bare essentials but can be extended later
on case by case. This way some people may even want to read it later on.

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>