clk: qcom: gcc-msm8974: switch from sleep_clk_src to sleep_clk

gcc-msm8974 uses the registered sleep_clk_src clock, which is just a 1:1
fixed factor clock register on top of the board's sleep_clk. Switch the
driver to use the board sleep_clk directly.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221228203725.3131237-1-dmitry.baryshkov@linaro.org