dt-bindings: clock: dispcc-sm8250: Add EDP_LINK_DIV_CLK_SRC index

Add this previously missing index, since it is supported by the SoCs
targeted by the dispcc-sm8250 driver.

Signed-off-by: Robert Foss <robert.foss@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221102090140.965450-4-robert.foss@linaro.org