KernelApiSysfsTest: make some nodes optional

The scaling_available_frequencies and time_in_state nodes may
not be available depending on the platform's cpufreq driver
support. Check the permissions and contents of these files only
if they are present.

Bug: 112194541
Test: run vts-kernel -m VtsKernelApiSysfs
Change-Id: Ibacfbdc0895d4688a9ca56f1067eedbb8757206d
Merged-In: Ibacfbdc0895d4688a9ca56f1067eedbb8757206d
(cherry picked from commit e897953affcffef9d5a430788876a02985188912)
1 file changed