clk: tegra: Refactor fractional divider calculation

Move this to a separate file so it can be used to calculate the sdmmc
clock dividers.

Signed-off-by: Peter De-Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>