dt-bindings: mfd: sprd: Add support for UMS9620

Add bindings for Unisoc UMS9620 system global registers which provide
register maps for clocks.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231215085630.984892-2-chunyan.zhang@unisoc.com
Signed-off-by: Lee Jones <lee@kernel.org>