clk: qcom: alpha-pll: Add support to adjust postdiv factor in slew ops

There could be a requirement where the PLL postdiv is
being set by either HW or SW. In those cases the
frequency of the PLL fout = fin/postdiv and the check
for VCO range would result in a failure. Adjust the
frequency with the post div value before the VCO range check.

Change-Id: I5cd556b5af7eb7936b157521096658aae4d1ebf3
Signed-off-by: Shefali Jain <shefjain@codeaurora.org>