MIPS: Alchemy: devboards: sit and spin after poweroff

On boards which don't support poweroff, systemd complains about this fact.
In case poweroff fails, just sit and spin in the wait loop.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Cc: Manuel Lauss <manuel.lauss@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/7558/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>