dm: timer: Convert to livetree

This updates dm_timer_init to support a live tree and deals with
some fallout (i.e. the need to restructure the code such, that we
don't need multiple discontinuous #if CONFIG_IS_ENABLED blocks).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>