Create the AON device when we realize the sifive_e machine.
This patch only implemented the functionality of the watchdog timer,
not all the functionality of the AON device.

Signed-off-by: Tommy Wu <tommy.wu@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230627141216.3962299-3-tommy.wu@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>