clk: qcom: clk-alpha-pll: Add support of slew recalc API

Add new API alpha_pll_slew_recalc_rate to correctly
calculate the rate for slew enabled PLL's.

Change-Id: I8428d99956d848b6c47a34d14dcf2b1ef9f8dc3a
Signed-off-by: Shefali Jain <shefjain@codeaurora.org>