clk: msm: clock: Update the bimc clock mux index

Use the ddrss_gcc_dbg_clk mux index to get the rate
measurement of the DDR clock on MSMCOBALT. Note that
this feature is only supported on v2 platforms.

CRs-Fixed: 1068034
Change-Id: Ib472c226b7e35079e6b8391848a0a0d1c38962c0
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>