dm: replace dm_dbg() with pr_debug()

As we discussed before in ML, dm_dbg() causes undefined reference
error if #define DEBUG is added to users, but not drivers/core/util.c

We do not need this macro because we can use pr_debug() instead, and
it is pretty easy to enable it for the DM core by using ccflags-y.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/drivers/core/Makefile b/drivers/core/Makefile
index 3d68c70..a5039c5 100644
--- a/drivers/core/Makefile
+++ b/drivers/core/Makefile
@@ -16,3 +16,5 @@
 obj-$(CONFIG_OF_CONTROL) += read.o
 endif
 obj-$(CONFIG_OF_CONTROL) += of_extra.o ofnode.o read_extra.o
+
+ccflags-$(CONFIG_DM_DEBUG) += -DDEBUG