clk: qcom: msm8996-cpu: Rename DIV_2_INDEX to SMUX_INDEX

The parent at this index is the secondary mux, which can connect
not only to primary PLL/2 but also to XO. Rename the index to SMUX_INDEX
to better reflect the parent.

Signed-off-by: Yassine Oudjana <y.oudjana@protonmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220621160621.24415-2-y.oudjana@protonmail.com