dt-bindings: mfd: sprd: Add bindings for ums512 global registers

Add bindings for Unisoc system global register which provide register map
for clocks.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Signed-off-by: Cixi Geng <cixi.geng1@unisoc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220909152421.278662-2-gengcixi@gmail.com