pmdomain: thead: create auxiliary device for rebooting

The reboot / power off operations require communication with the AON
firmware too.

As the driver is already present, create an auxiliary device with name
"reboot" to match that driver, and pass the AON channel by using
platform_data.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>