clk: qcom: Add support for post divider for debug mux

There could be cases where the post div in different debug muxes could be
set previously and not cleared, so add support to clear and set it, in case
required.

Change-Id: I15fedb4672179cb604804e7cbb0d6afc68bc473b
Signed-off-by: Taniya Das <tdas@codeaurora.org>