platform/x86/intel/pmc: Add debug attribute for Die C6 counter

platform/x86/intel/pmc: Add debug attribute for Die C6 counter

Add a "die_c6_us_show" debugfs attribute.  Reads the counter value using
Intel Platform Monitoring Technology (PMT) driver API. This counter is
useful for determining the idle residency of CPUs in the compute tile.
Also adds a missing forward declaration for punit_ep which was declared in
an earlier upstream commit but only used for the first time in this one.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20231129222132.2331261-20-david.e.box@linux.intel.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>