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>

tools/testing/selftests/cpufreq/.gitignore

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+cpufreq_selftest.*

tools/testing/selftests/cpufreq/Makefile

@@ -3,6 +3,7 @@ all:
 
 TEST_PROGS := main.sh
 TEST_FILES := cpu.sh cpufreq.sh governor.sh module.sh special-tests.sh
+EXTRA_CLEAN := cpufreq_selftest.dmesg_cpufreq.txt cpufreq_selftest.dmesg_full.txt cpufreq_selftest.txt
 
 include ../lib.mk