reboot: Use power off rather than busy spinning when halt is requested

Busy spinning after halt is dumb
We've previously applied this patch to arch/arm
but it is currenltly missing in arch/arm64

Pi4 after "sudo halt" uses 520mA
Pi4 after "sudo shutdown now" uses 310mA

Make them both use the lower powered option

Signed-off-by: Dom Cobley <popcornmix@gmail.com>