clk: fractional-divider: Improve approximation when zero based and export

Consider the CLK_FRAC_DIVIDER_ZERO_BASED flag when finding the best
approximation for m and n. By doing so, increase the range of valid
values for the numerator and denominator by 1.

Furthermore, export the approximation function so that users of this
function can be compiled as modules.

Cc: A.s. Dong <aisheng.dong@nxp.com>
Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
Link: https://lore.kernel.org/r/20230617131041.18313-2-frank@oltmanns.dev


Signed-off-by: Stephen Boyd <sboyd@kernel.org>