PPC: E500: Implement reboot controller

PPC: E500: Implement reboot controller

When Linux reboots an e500 VM, it writes to a magic register in the
"global-utilities" device indicated by the device tree. We were not
emulating that device so far, rendering the VM reboot-less.

This patch implements that device with only the reboot functionality
implemented and adds it to the device tree. With this patch applied,
I can successfully reboot a -M mpc8544ds VM.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Andreas Färber <andreas.faerber@web.de>