dm: core: Use U-Boot logging instead of pr_debug()

The pr_debug() functions do not response to setting the log level and in
fact have their own separate log level. Use U-Boot logging instead.

Perhaps we should make these options redirect to log_debug(), etc.?

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>