tools/power turbostat: be more useful as non-root

Don't exit if used this way:

sudo setcap cap_sys_nice,cap_sys_rawio=+ep ./turbostat
sudo chmod +r /dev/cpu/*/msr
./turbostat

note: cap_sys_admin is now also needed for the perf IPC counter:
sudo setcap cap_sys_admin,cap_sys_nice,cap_sys_rawio=+ep ./turbostat

Reported-by: Artem S. Tashkinov <aros@gmx.com>
Reported-by: Toby Broom <tbroom@outlook.com>
Signed-off-by: Len Brown <len.brown@intel.com>