dt-bindings: watchdog: Add ast2600 compatible

This adds a compatible for the ast2600, a new ASPEED SoC.

Signed-off-by: Joel Stanley <[e-mail]>
Reviewed-by: Andrew Jeffery <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Guenter Roeck <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

Documentation/devicetree/bindings/watchdog/aspeed-wdt.txt

···
  - compatible: must be one of:
 	- "aspeed,ast2400-wdt"
 	- "aspeed,ast2500-wdt"
+	- "aspeed,ast2600-wdt"
 
  - reg: physical base address of the controller and length of memory mapped
    region