msm: clock-a7: Add safe-freq property for freq switch

Safe frequency variable is used for switching from freq
A->B, and currently it is always fixed at 300MHz.
Use this property(optional) to override the value which
is supported on the device for switch.

Change-Id: I59d89ff2c4caaa108c9c6fc73d325e9a585b57e3
Signed-off-by: Taniya Das <tdas@codeaurora.org>