selftests: intel_pstate: notification about privilege required to run intel_pstate testing script

The intel_pstate related testing script need root level privileges
when trying to access certain file for the successful execution of
the script.But this is not the case always like when using evaluation
only mode, which only require user level privilege.

This patch is to notify the user about the privilege the script
demands for the successful execution of the test.

Signed-off-by: Jeffrin Jose T (Rajagiri SET) <ahiliation@gmail.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>