tools/power/x86/intel-speed-select: Add check for CascadeLake-N models

Three CascadeLake-N models (6252N, 6230N, and 5218N) have SST-PBF support.

Return an error if the CascadeLake processor is not one of these specific
models.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>