platform/x86/intel/tpmi: Add API to get debugfs root

Add new API to get the debugfs root directory for TPMI. This allows any
TPMI devices to add their own debugfs items under the same directory
structure.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
Link: https://lore.kernel.org/r/20240527133400.483634-3-tero.kristo@linux.intel.com


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