tools/power/x86/intel-speed-select: Introduce punit to isst_id

Punit id can also be retrieved from ISST_IF_GET_PHY_ID.

punit id is unique within a Package/Die, and together with Package id and
Die id, they can be used to refer to a specific SST power domain.

For current platforms, Punit id is always Zero. So no functional changes
are expected for the current platforms.

While here, prevent issuing IOCTL if the file /dev/isst_interface can't be
opened.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[srinivas.pandruvada@linux.intel.com: changelog edits]
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>