clk: qcom: Add support to be able to slew PLL

Few alpha PLLs would require to be able to slew in the same VCO mode, which
means the PLL would be able to update the new frequency L value without
turning it off. But to be support this feature the PLL needs to calibrated
at the mid of the VCO range and then enabled.

Change-Id: I24820bd6254002f8a8db9604d230dcbce59b1beb
Signed-off-by: Taniya Das <tdas@codeaurora.org>