clk: spacemit: introduce pre-div for ddn clock

The original DDN operations applied an implicit divide-by-2, which should
not be a default behavior.

This patch removes that assumption, letting each clock define its
actual behavior explicitly.

Reviewed-by: Haylen Chu <heylenay@4d2.org>
Signed-off-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>