Add more Aspeed watchdog registers from [*].

Since guests can righteously access them, log the access at
'unimplemented' level instead of 'guest-errors'.

[*] https://github.com/AspeedTech-BMC/zephyr/blob/v00.01.08/drivers/watchdog/wdt_aspeed.c#L31



Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Delevoryas <peter@pjd.dev>
Signed-off-by: Cédric Le Goater <clg@kaod.org>