dt-bindings: watchdog: dw-wdt: add description for rk3568

add "rockchip,rk3568-wdt", "snps,dw-wdt" for watchdog nodes on
a rk3568 platform to snps,dw-wdt.yaml.

Signed-off-by: Liang Chen <[e-mail]>
Acked-by: Rob Herring <[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/snps,dw-wdt.yaml

···
               - rockchip,rk3328-wdt
               - rockchip,rk3368-wdt
               - rockchip,rk3399-wdt
+              - rockchip,rk3568-wdt
               - rockchip,rv1108-wdt
           - const: snps,dw-wdt