dt-bindings: mfd: syscon: Add StarFive JH7100 sysmain compatible

Document StarFive JH7100 SoC compatible for sysmain registers.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230211031821.976408-7-cristian.ciocaltea@collabora.com