selftests/cpufreq: Fix cpufreq basic read and update testcases

selftests/cpufreq: Fix cpufreq basic read and update testcases

In cpufreq basic selftests, one of the testcases is to read all cpufreq
sysfs files and print the values. This testcase assumes all the cpufreq
sysfs files have read permissions. However certain cpufreq sysfs files
(eg. stats/reset) are write only files and this testcase errors out
when it is not able to read the file.
Similarily, there is one more testcase which reads the cpufreq sysfs
file data and write it back to same file. This testcase also errors out
for sysfs files without read permission.
Fix these testcases by adding proper read permission checks.

Link: https://lore.kernel.org/r/20250430171433.10866-1-swapnil.sapkal@amd.com
Reported-by: Narasimhan V <narasimhan.v@amd.com>
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>