clk: msm: clock-local: Add support to set the post_div for debug mux

Update the clock framework to support the setting of post_div for
debug mux so as to divide the clock by post_div.

CRs-Fixed: 977413
Change-Id: I7299bdb0953dcf65fbf2a38b7578e2e54446c0d7
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>