clk: fractional-divider: Document the arithmetics used behind the code

It appears that some code lines raise the question why they are needed
and how they are participated in the calculus of the resulting values.

Document this in a form of the top comment in the module file.

Reported-by: Liu Ying <[email protected]>
Signed-off-by: Andy Shevchenko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[[email protected]: Remove "die" as it isn't relevant]
Signed-off-by: Stephen Boyd <[email protected]>

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