watchdog: aspeed: Sanitize control register values

watchdog: aspeed: Sanitize control register values

While some of the critical fields remain the same, there is variation in
the definition of the control register across the SoC generations.
Reserved regions are adjusted, while in other cases the mutability or
behaviour of fields change.

Introduce a callback to sanitize the value on writes to ensure model
behaviour reflects the hardware.

Fixes: 854123bf8d4b ("wdt: Add Aspeed watchdog device model")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20210709053107.1829304-2-andrew@aj.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>