dm: core: migrate debug() messages to use dm_warn

Prior to that, seeing the debug() messages required to enable DM_DEBUG
which defines DEBUG (and then _DEBUG) which in turn makes failing
assert() calls reset U-Boot which isn't necessarily what is desired.

Instead, let's migrate to dm_warn which is using log_debug when unset or
log_warn when set.

While at it, reword the DM_DEBUG symbol in Kconfig to explain what it
now actually does.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>