clk: qcom: Consolidate frequency finding logic

There are two find_freq() functions in clk-rcg.c and clk-rcg2.c
that are almost exactly the same. Consolidate them into one
function to save on some code space.

Cc: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>