doc: TPMI: Add debugfs documentation

Describe fields in the TPMI debugfs folder.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230712225950.171326-4-srinivas.pandruvada@linux.intel.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>