clk: fractional-divider: Export approximation algorithm to the CCF users

clk: fractional-divider: Export approximation algorithm to the CCF users

At least one user currently duplicates some functions that are provided
by fractional divider module. Let's export approximation algorithm and
replace the open-coded variant.

As a bonus the exported function will get better documentation in place.

Signed-off-by: Andy Shevchenko <[email protected]>
Tested-by: Heiko Stuebner <[email protected]>
Acked-by: Heiko Stuebner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[[email protected]: Add header guard because why not]
Signed-off-by: Stephen Boyd <[email protected]>