qcom: clock-generic: Introduce a "safe" divider value

Some clock tree implementations have a mux that needs
to be switched to a safe source before reprogramming
an upstream source. A downstream divider would also
have to switch to a "safe" divider value to allow for
that safe frequency to be generated.

Change-Id: Id1892501e97cac77e6d89bef7fcd44f2944027a8
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>