ARM: shmobile: r8a7779: Stop powering down secondary CPUs during early boot

At .smp_prepare_cpus() time, CPUs 1-3 are still powered down:
  - The bootloader doesn't enable CPUs 1-3,
  - Non-boot CPUs are disabled by kexec before starting the new kernel.

Hence there is no need to try to power them down again, as it will fail
silently with -EIO anyway.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>