clk: twl: remove is_prepared

Remove is_prepared to simplify adding of TWL6030 support.
The default implementation should be enough.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20241014161109.2222-3-andreas@kemnade.info
Signed-off-by: Stephen Boyd <sboyd@kernel.org>