tools/power/cpupower: Display boost frequency separately

cpufreq driver creates sysfs file "scaling_boost_frequency" for platforms
which support boost frequency. Cpupower now prints boost frequencies
separately. For few x86 vendors who already have different way to get boost
frequency, will continue to use the existing logic. Rest of the platforms
will rely on "scaling_boost_frequency" file to display boost frequency.

Signed-off-by: Abhishek Goel <huntbag@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>