arm64: dts: uniphier: add watchdog node for LD11 and LD20

Add nodes of watchdog timer for UniPhier LD11 and LD20 SoC.
The watchdog timer is included in sysctrl.

Signed-off-by: Keiji Hayashibara <hayashibara.keiji@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>