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/Kconfig b/drivers/core/Kconfig
index 7afef1f..e8ba20c 100644
--- a/drivers/core/Kconfig
+++ b/drivers/core/Kconfig
@@ -45,6 +45,12 @@
 	  This will cause dm_warn() to be compiled out - it will do nothing
 	  when called.
 
+config DM_DEBUG
+	bool "Enable debug messages in driver model core"
+	depends on DM
+	help
+	  Say Y here if you want to compile in debug messages in DM core.
+
 config DM_DEVICE_REMOVE
 	bool "Support device removal"
 	depends on DM