um: register power-off handler

Otherwise we always get

 reboot: Power off not available: System halted instead

which is really quite pointless.

Link: https://patch.msgid.link/20240703173839.fcbb538c6686.I3d333f4773cff93c4337c4d128ee0b1b501b3dfa@changeid


Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>