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 <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210429081151.17558-5-cl@rock-chips.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>