ipq806x: add missing scaling_available_frequencies for dedicated cpufreq

ipq806x: add missing scaling_available_frequencies for dedicated cpufreq

Add missing scaling_available_frequencies sysfs entry for dedicated
cpufreq driver.
This sysfs entry is not standard and each cpufreq driver needs to
provide it and declare it in the cpufreq driver struct attr.

Fixes: 5dbbefcbccc0 ("ipq806x: introduce dedicated krait cpufreq")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
[ fix conflict by dropping 5.15 patch not present in openwrt-22.03 ]
(cherry picked from commit d6994c53cd0e5334478d2aab424674da438804e2)