clk: msm: Model mss_cfg_ahb_clk as a branch clock

On mdmcalifornium this clock needs to be modelled as a branch clock
instead of an RPM clock.

Change-Id: I5dface0660349d865182e291ceadfb69b642bd39
Signed-off-by: Devesh Jhunjhunwala <deveshj@codeaurora.org>