tools/power/x86/intel-speed-select: Add new command turbo-mode

Add a new command "turbo-mode", which allows to enable/disable
turbo mode globally. This uses base-frequency as the max frequency
when turbo-mode is disabled. This allows soft disable turbo mode
without depending on kernel or BIOS.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>