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/[e-mail]/
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: Viresh Kumar <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Li Zhijian <[e-mail]>
Acked-by: Viresh Kumar <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/cpufreq/.gitignore

···
+# SPDX-License-Identifier: GPL-2.0-only
+cpufreq_selftest.*

tools/testing/selftests/cpufreq/Makefile

···
 
 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