dt-bindings: serial: Add a new compatible string for UMS9620

The UMS9620 also uses the same serial device with SC9836.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lore.kernel.org/r/20231215085630.984892-4-chunyan.zhang@unisoc.com


Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>