tools/power/x86/intel-speed-select: Switch output to MHz

These features are introduced on new processors that will never operate
in the KHz range.

Save some zeros and switch the output to MHz.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: David Arcari <darcari@redhat.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>