dm: include <dm/util.h> from drivers/core/util.c

Fix sparse warnings "... was not declared. Should it be static?"

Also, fix redefinition of dm_warn/dm_dbg.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>