dt-bindings: clk: qcom: gcc-sm6125: Document SM6125 GCC driver

Document the newly added SM6125 GCC driver.

Signed-off-by: Martin Botka <martin.botka@somainline.org>
Link: https://lore.kernel.org/r/20210605121040.282053-1-martin.botka@somainline.org
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>