configs/raspberrypi*: stop setting powersave as the default CPU governor

[clopez]

• 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 buildroot 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.

• 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 one of: ondemand, schedutil or performance depending on the Kernel version and the hardware. For modern Kernels (>v5.6) schedutil is the one preferred for ARM hardware. See Linux Kernel commit: https://git.kernel.org/linus/f259eab3ea0e7e

• This patch was submitted upstream (both to the RPi kernel defconfigs and to upstream buildroot but was not accepted). See: https://lists.buildroot.org/pipermail/buildroot/2023-October/677641.html and configs: stop defaulting to the powersave governor raspberrypi/linux#5666

• This PR already updates all RPi defconfigs, but in the future if new defconfigs are added for RPi boards then this command can be used to automate updating them:

for f in configs/raspberrypi*_defconfig; do
  if ! grep -q "board/raspberrypi/default-cpu-governor.fragment" "$f"; then
    sed -i 's,BR2_LINUX_KERNEL=y,BR2_LINUX_KERNEL=y\nBR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="board/raspberrypi/default-cpu-governor.fragment",' "$f"
  fi
done