driver: reset: th1520-aon: add driver for poweroff/reboot via AON FW

This driver implements poweroff/reboot support for T-Head TH1520 SoCs
running the AON firmware by sending a message to the AON firmware's WDG
part.

This is a auxiliary device driver, and expects the AON channel to be
passed via the platform_data of the auxiliary device.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Acked-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>