enable cpufreq and add cpuidle workaround
Apply the current workaround by NXP to enable the "cpu-sleep" C-state for cpuidle to use. See https://lkml.org/lkml/2019/6/10/350
This needs u-boot built using the corresponding ATF changes: Librem5/trusted-firmware-a!2 (merged)
This adds some (cpufreq) changes that currently already are in linux-next but not yet in v5.2.