tools/power/x86/intel-speed-select: Abstract get_coremask_info

Allow platform specific implementation to get the core mask for a given
SST-PP level.

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>