dt-bindings: mailbox: qcom: correct the list of platforms using clocks

Only three platforms require `pll' and `aux' clocks: msm8916, msm8939
and qcs404. Correct the list of platforms in the corresponding clause.

Fixes: 0d17014e ("dt-bindings: mailbox: Add binding for SDX55 APCS")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>