dt-bindings: soc: socionext: Add UniPhier system controller

Add devicetree binding schema for the system controller implemented on
Socionext Uniphier SoCs.

This system controller has multiple functions such as clock control,
reset control, internal watchdog timer, thermal management, and so on.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221213082449.2721-10-hayashi.kunihiko@socionext.com


Signed-off-by: Rob Herring <robh@kernel.org>