selftests/cpufreq: gitignore output files and clean them in make clean

selftests/cpufreq: gitignore output files and clean them in make clean

After `make run_tests`, the git status complains:
Untracked files:
    (use "git add <file>..." to include in what will be committed)
        cpufreq/cpufreq_selftest.dmesg_cpufreq.txt
        cpufreq/cpufreq_selftest.dmesg_full.txt
        cpufreq/cpufreq_selftest.txt

Link: https://lore.kernel.org/all/20241122074757.1583002-1-lizhijian@fujitsu.com/
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>