tpm: update PPI documentation to address the location change.

Updated Documentation/ABI/testing/sysfs-driver-ppi in order to explain
where PPI attributes are located and how backwards compatibility is
addressed.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>