dt-bindings: watchdog: fsl,scu-wdt: Document imx8dl

imx8dxl also contains the SCU watchdog block.

Add an entry for 'fsl,imx8dxl-sc-wdt'.

Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20231004182043.2309790-1-festevam@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>