clk: qcom: clock-generic: Add a field that indicates a low power source

Implementations may want to differentiate between the
safe source that is used when switching frequencies and
the one used when switching to low power modes. Add a field
to provide this flexibility and alleviating the need for
implementation specific ops.

Change-Id: Icd9ac9d11f0e39a06fe0f70531b3576e888afb2d
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>