cpupower: Add 'help' target to the main Makefile

Make "cpupower" building process more user friendly by adding 'help'
target to the main makefile. This target describes various build
and cleaning options available to the user.

Signed-off-by: Roman Storozhenko <romeusmeister@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>