tools/power/x86/intel-speed-select: Introduce api_version helper

In some cases, the output format may be different with different
api_version because of different capabilities or for backward
capabilities reason.

Introduce api_version() to get the api_version of the platform running.

No functional changes are expected.

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>