src/meta-raspberrypi:linux-raspberrypi: stop setting powersave as the…

… default CPU governor

* The defconfigs from the RPi Kernel set `powersave` as the default
  CPU governor, which is a bad idea as that reduces performance by
  setting the CPU frequency to the minimum one.

* In case of a Yocto build user-space is not configured by default
  to change the CPU governor, so `powersave` will remain the CPU
  governor and it will slow down everything.

* I submitted a fix for those defconfigs to upstream here:
  raspberrypi/linux#5666
  However, we shouldn't wait until that is accepted and backported
  to all the stable branches (if ever is).

* This patch unsets the selection of `powersave` as the default CPU
  governor.

* Then the CPU governor that will be selected as default after this
  patch will be the Linux's default one that is either `ondemand` or
  `schedutil` depending on the Kernel version.

Signed-off-by: Carlos Alberto Lopez Perez <[email protected]>