Some dividers might have a maximum value that is lower than the width of
the register.

Add a field to _ccu_div to handle those case properly. If the field is set
to 0, the code will assume that the maximum value is the maximum one that
can be used with the field register width.

Otherwise, we'll use whatever value has been set.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>