tools/power/x86/intel-speed-select: Read TRL from mailbox

When SST-PP feature is not present, the TRL (Turbo Ratio Limits)
is read from MSRs. This is done as the mailbox command will fail
on Skylake-X based platform. But for IceLake servers, mailbox
commands can still be used. So add a check to allow for non Skylake
based platforms to read from mail box commands.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://lore.kernel.org/platform-driver-x86/57d6648282491906e0e1f70fe3b9a44f72cec90d.camel@intel.com/


Signed-off-by: Hans de Goede <hdegoede@redhat.com>