watchdog: kconfig: Enable designware for rk3399

Enable designware watchdog driver for rk3399 if WDT defined.

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>