MIPS: Loongson64: reset: Prioritise firmware service

MIPS: Loongson64: reset: Prioritise firmware service

We should always use firmware's poweroff & reboot service
if it's available as firmware may need to perform more task
than platform's syscon etc.

However _machine_restart & poweroff hooks are registered at
low priority, which means platform reboot driver can override
them.

Register firmware based reboot/poweroff implementation with
register_sys_off_handler with appropriate priority so that
they will be prioritised. Remove _machine_halt hook as it's
deemed to be unnecessary.

Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>