arm64: dts: uniphier: add nodes of thermal monitor and thermal zone for LD20

Add nodes of thermal monitor and thermal zone for UniPhier LD20 SoC.
The thermal monitor node is included in sysctrl. Since the efuse might not
have a calibrated value of thermal monitor, this patch gives the default
value for LD20.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>